ISSN 10637745, Crystallography Reports, 2014, Vol. 59, No. 3, pp. 331–337. © Pleiades Publishing, Inc., 2014. Original Russian Text © A.A. Lomov, A.V. Myakonkikh, K.V. Rudenko, Yu.M. Chesnokov, 2014, published in Kristallografiya, 2014, Vol. 59, No. 3, pp. 374–380.
DIFFRACTION AND SCATTERING OF IONIZING RADIATIONS
XRay Reflectometry of the Specific Features of Structural Distortions of He+Implanted Si(001) Surface Layers A. A. Lomova, A. V. Myakonkikha, K. V. Rudenkoa, and Yu. M. Chesnokovb a
Institute of Physics and Technology, Russian Academy of Sciences, Moscow, Russia email:
[email protected] b National Research Centre “Kurchatov Institute”, pl. Akademika Kurchatova 1, Moscow, 123182 Russia Received June 13, 2013
Abstract—The structural changes in the surface layers of silicon substrates, implanted by helium ions with energies from 2 to 5 keV and doses to 6 × 1015–5 × 1017 cm–2, has been studied by highresolution Xray reflectometry. The damaged layer is found to have a total thickness comparable with the total ion path length (estimated from the SRIM model) and a multilayer structure: a strongly amorphized layer with reduced den sity, a porous (incapsulated) layer, and a deformed layer. The thickness of sublayers, their density ρ(z), and the mean strain (∼5 × 10–3) have been determined. The characteristic pore size is estimated to be 5–20 nm. It is shown that the presence of a nanoporous layer facilitates the formation of diffuse scattering, which can be used to diagnose layers by highresolution Xray reflectometry. DOI: 10.1134/S1063774514020138
INTRODUCTION The ion implantation of silicon has been applied for a long time as a technique for modifying its electri cal properties and structure near the crystal surface [1]. The incorporation of atoms into the crystal lattice is accompanied by the formation of many radiation defects in it (vacancies, interstitials, etc.), which change the electrical and mechanical properties of layers. When studying implanted layers, particular attention is given to the degree of their disordering, i.e., amorphization. One urgent problem of modern technology is the formation of a completely amor phized layer on the silicon surface, which impedes the channeling of small impurity ions during their subse quent implantation into silicon [1, 2]. In other cases, the implantation of light ions (for example, protons) to high doses leads to the formation of buried (incap sulated) nanoporous silicon layers, which play an important role in SmartCut technologies aimed at designing silicon structures on an insulator [1]. The interest in these layers with a small location depth is also related to their luminescence [3–5] and photovol taic properties [6]. One widespread nondestructive method for moni toring the structural quality of implanted layers is Xray diffraction in the Bragg geometry. Diffraction broadening hinders the application of standard Xray techniques to analyze nanoscale layers formed by the implantation of (0.5–5)keV ions. In addition, in the case of highdose implantation, significant radiation damage in the surface layer leads to interference extinction and, as a consequence, to the attenuation or even disappearance of diffraction reflections from
it. In this case one can efficiently use techniques based on Xray grazing incidence diffraction from a sample. However, these methods require high collimation and highpower Xray sources. The highdose implantation of silicon by protons and helium ions leads to its amorphization and intense pore formation in the implanted layer. These processes were observed previously [7]; however, no detailed Xray diffraction study of silicon layers implanted by lowenergy helium ions was performed. In this case, along with occurrence of structural lattice distortions, the density of the material significantly changes, which can be detected and studied by highresolution Xray reflectometry (XRR). This study is devoted to the development of an XRR method for diagnosing the structure, thickness, and density of silicon surface layers subjected to high dose implantation by lowenergy helium ions. To elab orate a more complete model of surface layers in order to describe their total thickness, strain, and structural distortions, we applied the rocking curve method. EXPERIMENTAL Samples 30 × 40 mm in size were cleaved out from standard Si(100) KDB12 (pSi:B with a resistance of 12 Ω cm) substrates. Directly before implantation, a layer of native oxide was removed from the surface by etching it in a 5% solution of hydrofluoric acid at room temperature for 1 min. The implantation of He+ ions was performed in a plasmaimmersion lowvoltage ion implanter (developed at the Institute of Physics and Technology, Russian Academy of Sciences), equipped with an inductively coupled plasma (ICP) source. The
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Table 1. Energies E, doses N of He+ ions implanted into Si(001), and thicknesses of surface sublayers with violations of different types: (LA) amorphized, (LP) porous, and (LD) defect layers in their sequence order with respect to the sample surface Sample E, keV
of the rockingcurve tail. In the XRR experiments, the size of the monochromator output slit was 20 μm in the horizontal plane and 2 mm in the vertical plane. The slit mounted before the detector had an angular width of 140″.
TEM
N, cm–2
RESULTS AND DISCUSSION
LA, nm LP , nm LD , nm N1EI N2EI N3EI
2 2 5
6.0 × 10 1.2 × 1016 5.0 × 1017
15
5.0(5) 7.5(5) ~10
~17
~8 90(5)
helium pressure in the working chamber was main tained at a level of 10 mTorr at a discharge power of 500 W. Implantation was performed by applying rect angular pulses of negative accelerating potential of 2– 5 kV with a width of 10 μs and a frequency of 1 kHz to samples. The implantation dose (Table 1) was deter mined by measuring the ion current through the sam ple during pulsed irradiation. An Xray study was performed on an automatic TRS diffractometer (Special Design Bureau of the Institute of Crystallography, Russian Academy of Sci ences). An Xray beam (СuKα1 radiation) was formed using a channelcut 3fold reflecting Ge(004) mono chromator. XRR curves were recorded in the ω/2θ scan geometry, while the total scattering cross sections were recorded in the ωscan geometry for the sample with a detector installed in a fixed position. Si(400) rocking curves were measured for parallel orientation of the monochromator and sample crystals. Disper sion did not exceed 15″ and did not affect the behavior
Before carrying out heliumion implantation, the surface state of initial silicon substrates was monitored by scanning probe microscopy (SPM) on a Nanopics 2100 system (KLATencor). At a scan size of 5 × 5 μm, the surface microroughness was found to be σrms = 0.20(5) and σа = 0.20(5) nm for the initial substrate after removing native oxide layers with thicknesses of 0.70(5) and 0.40(5) nm, respectively. To compare the obtained values of surface roughness, we measured the specular reflection and analyzed the data obtained using the programs reported in [8, 9]. One example of the experimental XRR curve from the initial substrate after chemical cleaning is shown by filled circles (curve 3) in Fig. 1. A simulation based on visual coin cidence [8] and fitting (χ2 = 1.0) of XRR curves using the technique [9, 10] showed the presence of a transi tion layer (correspondingly, profiles 1 and 2 in Fig. 3) with a thickness of ∼0.8 nm. Note that the model (pro file 1) includes also the roughness σ = 0.3 nm at the interface between the surface layer and silicon. Physi cally, this means some spread of electron density in this region. With allowance for small thicknesses of transition layers, these profiles can be considered coinciding. In the χ2 method, the most reliable parameters of layers are reconstructed when this crite rion tends to unity. This good coincidence of the sur
log(I/I0) 2
0 2 –2
3
–4
1 –6 0
0.5
1.0
1.5
2.0 ϑ, deg
Fig. 1. Experimental (vertical marks, dashed line) and theoretical (solid lines) XRR curves for samples (1) N1EI and (2) N2EI and for (3) the initial substrate. CRYSTALLOGRAPHY REPORTS
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face layer parameters, reconstructed from XRR data using different techniques, can be implemented for only the surfaces that are close to ideal. In the case of layers with a nontrivial density distribution in the sur face layer, simulation based on program [8] was not successful because of the absence of a criterion for selecting reliable solutions. The deflections and macrodistortions of the sample surface were characterized on a threedimensional optical interference analyzer of surface morphology ZYGO New View (ZYGO, United States); the instru ment had a resolution of 0.3 nm along the normal to the surface. Each measurement was performed on an area of 1.2 × 1.2 mm; the standard deviation of the roughness heights Rrms and their means Rа were deter mined for each area, with the subsequent averaging of measurement results over three areas. The measure ment results are listed in Table 2. Visually, the implanted samples had a mirror surface with a slightly pronounced metallic blaze, as compared with the ini tial substrate; this feature is characteristic of amor phized silicon. An analysis of the data of Table 2, with allowance for the SPM data, shows that, on the whole, implanta tion by (2–5)keV helium ions did not lead to an increase in the surface roughness; moreover, the sur face of sample N2EI sample became even smoother. Therefore, we can consider the vacuum–crystal inter face under our conditions as invariable in comparison with the initial sample. The XRR curves from the samples under study are shown in Figs. 1 and 2 on the logarithmic scale. It can be seen that implantation to a dose of 6.0 × 1015 cm–2 leads to a significant change in the curve shape with respect to the XRR curve from the initial substrate
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Table 2. Parameters of the macroroughness of the sample surface according to the data of optical contactless structure analyzer (ZYGO, United States) Samples Measured parame initial substrate af N1EI ters substrate ter cleaning Rrms , nm Rа, nm
2.6(2) 2.0(2)
2.5(2) 2.0(2)
N2EI
N3I
2.3(2) 1.5(2) 2.0(2) 2.0(2) 1.2(2) 2.0(2)
(Fig. 1, curve 3). Model calculations showed that the curve “deflection” is due to the presence of a layer with reduced electron density near the surface. Curve 2 from sample N2EI (compare with curve 1) exhibits similar but more pronounced features. Therefore, an increase in the implantation dose to 1.2 × 1016 cm–2 did not change the electron density distribution in the surface layer and only affected its numerical parame ters. This does not hold true for the XRR curve (Fig. 2) from sample N3EI. This curve exhibits oscillations of two types: (1) pronounced oscillations near the critical angle ϑС and (2) aperiodic oscillations at angles exceeding ϑС. The oscillations of group 1 have a reso nant nature and indicate the presence of a layer with reduced polarizability in the sample. A similar effect can be observed in a Fabry–Perot resonator. To reconstruct the relative electron density profile ρ(z) for the surface layers of samples, we performed the fitting of theoretical XRR curves to experimental ones using the technique proposed in [9]. The real geometry of the experiment, the measurement error, and the contribution of diffuse scattering from density irregularities were taken into account. The implanted
log(I/I0) 0 –1 –2 –3 –4 –5 –6 0
0.2
0.4
0.6
0.8
1.0 ϑ, deg
Fig. 2. Experimental (vertical marks) and theoretical (solid lines) XRR curves for sample N3EI. The inset shows part of the exper imental curve in the vicinity of the angle ϑC on the linear scale. CRYSTALLOGRAPHY REPORTS
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Si
1.0
Si
(a)
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ρ(Z)/ρ0
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2
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1
0
2
0
4
6
8 Si
Si
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2
(d)
0.5
0
5
10
15 20 z, nm
25
30
0
20
40
60 z, nm
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100
Fig. 3. Electron density profiles for the surface layers of (a) the initial substrate (curves 1 [8] and 2 [9]) and samples (b) N1EI, (c) N2EI, and (d) N3EI reconstructed within the step model with linear transition layers. The dashed lines show the error corridor.
layer was modeled by a set of sublayers of thickness li with a polarizability χ0i and transition layers of thick ness Δli, with polarizabilities linearly changing between layers. Within the model chosen, we obtained the following χ2 values: 1.6, 2.9, and 3.7 for samples N1EI, N2EI, and N3EI, respectively. Note that the theoretical XRR curves describe the experimental ones for samples N1EI and N2EI fairly well. In regards to sample N3EI, the distribution density in its surface layer can be considered correct in only the first approximation, because we failed to describe all fine features (Fig. 2, inset) in the experimental reflectom etry curve. The density distributions in implanted samples, calculated within the model chosen, are shown in Fig. 3. An analysis of their shape indicates that the density of the material at the vacuum–substrate inter face gradually increases. The upper sublayer becomes denser with an increase in both the dose and energy of ions. Note a rather low (with respect to the bulk mate rial) density of the upper sublayer in a fairly wide range of thicknesses. Attempts to measure the surface rough ness by the SPM method were unsuccessful because the surface was destructed by the probe; this fact is an indirect evidence of low density of the material. To determine the real values and confirm the XRR data, one must perform additional studies. Sharp density jumps at the interface between the layer and bulk sili
con are likely to be nonphysical; in the framework of experimental data, they can be replaced with ∼1nm thick transition layers. However, this replacement did not change the main part of the density distribution. A characteristic feature of the ρ(z) profiles for the samples implanted to high doses is the formation of an inverse (with a “dip” in density) sublayer for sample N2EI; the XRR curve for this sample (Fig. 1) does not differ much from that for sample N1EI. The most pro nounced “dip,” both in the largest difference in densi ties and in the inversesublayer thickness, is observed for sample N3EI. The attempts to elaborate a true model of the layer, ρ(z), without a dip and with a χ2 value equal to the found one or smaller, were not suc cessful. This result is in agreement with the data of [11]. The implantation of helium ions to large doses leads (due to the coagulation of vacancies in the sur face layer of amorphized silicon) to the formation of pores which are initially filled with helium. As a result, an incapsulated nanoporous layer with a lower density is formed. The porous layers considered here, in con trast to the porous silicon formed by anodic oxidation, are formed in the bulk of the surface region of silicon substrates. The layers and their boundaries have spe cific structural features, which may radically change their properties [5–7, 13]. The pore structure should depend on the mechanism of substrate radiation dam age, the properties of accumulated impurity helium,
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335
CHe, cm–3
2.5 2.0
1022
1.5 1.0 1021
0.5 0 –0.5
1020 0.1
0.2
0.3
0.4
0.5
0.6
0.7 ω, deg
0
Fig. 4. Experimental ωscan curves from (䊊) the initial substrate, (䊉) the N1EI sample, and (solid line) the N3EI sample.
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150 z, nm
Fig. 5. Calculated profiles of Heion concentration in Si(001) for samples (䊐) N1EI, (䊏) N2EI, and (䊊) N3EI.
and the kinetics of radiationinduced defects both during implantation and after it [13]. It is possible that reconstructed electron density profiles do not com pletely describe the real state of material in the incap sulated layer; however, the data obtained reliably indi cate its formation and kinetics, as well as the effect of “compacting” the upper sublayer from the side of the layer incapsulated at higher doses. The formation of an incapsulated porous layer should lead to the occurrence of diffuse scattering at Xray grazing angles exceeding the XRR critical angle. To verify this suggestion, we recorded ωscan curves (Fig. 4) at a fixed detector position (0.832 arc s). It can be seen that the implantation of helium ions to a dose of 5.0 × 1017 cm–2 gives rise to additional scattering in the form of Yoneda peaks at both sides of the specular peak, located at 0.416 arc s. Diffuse scattering at spec ular reflection from porous gallium arsenide layers was observed in [14]. The pore parameters were deter mined in [15] by analyzing the diffuse scattering inten sity from bulk density inhomogeneities with allowance for the contribution of surface roughness. In this study the initial and implanted samples are characterized by the same surface roughness, which does not form significant diffuse scattering. Therefore, the occurrence of diffuse scattering peaks in the curve from sample N3EI unambiguously indicates the for mation of a porous sublayer in the surface region. With allowance for the data on ρ(z) and the model of pores, we can suggest that the latter should be 5–20 nm in size. One of the main parameters of the model describ ing layers is their thickness, i.e., the part of the sample where structural distortions can be revealed by the technique we used. To compare the thicknesses of lay CRYSTALLOGRAPHY REPORTS
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ers with possible structural distortions, we calculated the heliumion distribution profiles (Fig. 5) for the samples under study using the SRIM program [16]. As we suggested, a comparison of the calculated profiles of Heion concentration, СНе(z), with the layer density profiles ρ(z) reconstructed from reflectometry data (Fig. 3), showed that the thicknesses of layers with sig nificant structural distortions must coincide. Indeed, a question arises about the difference in the layer thicknesses (Fig. 3) (found based on the XRR data) for samples implanted by 2keV ions to different doses. For sample N2EI, the XRR and SRIM data can be considered coinciding, whereas for sample N1EI they differ by a factor of more than 2. At the same time, the XRR data correlate well with the thickness of damaged layers analyzed by the TEM method [17] (Table 1). At a low implementation dose, the concen tration of violations in depth of the surface layer (at the interface with the bulk material) does not lead to a change in density and does not manifest itself in the XRR curves and TEM images. An additional study by the rocking curve method revealed the existence of deformed layers with an enlarged lattice parameter (Δd/d ∼ 5 × 10–3) in the sur face region of the samples. The deformed regions (up to ∼100 nm thick) are located under highly amor phized layers. The deformedlayer thickness for sam ple N1EI exceeds that for N2EI, which is indicative of its reduction with an increase in the implantation dose. As this study showed, this occurs partially due to the stress relaxation at the incapsulated layer– deformed layer interface. Thus, the total thickness of the layer damaged as a result of implantation by 2keV He ions remains constant. The above considerations explain the contradiction between the thicknesses of
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layers and a detailed consideration of diffuse scatter ing, which yields the main information about pore parameters. CONCLUSIONS
1 μm Fig. 6. TEM image (in transmission) of a portion of the surface of wedgeshaped sample N3EI.
layers with a changed density and the mean path length of implanted ions. An additional TEM study [17] of implanted sam ples showed that the highdose implantation of He ions into silicon leads to the formation of layers with different structures in the surface region of the samples (Table 1): an amorphous layer LA (native oxide and amorphous silicon); an incapsulated layer LP (porous silicon); and a crystalline silicon layer LD with a high content of structural defects, the density of which decreases in the direction toward the sample bulk. The structural parameters of these layers depend strongly on the implantation conditions; therefore, they are differently revealed and determined by particular ana lytical methods. To reveal pores near the surface, we prepared a wedgeshaped sample. This sample (N3EI) was inves tigated at an accelerating voltage of 300 keV in a TITAN 80300 transmission scanning electron micro scope (FEI, United States) equipped with a probe spherical aberration corrector in bright and dark field modes. The TEM image (Fig. 6) of a surface area reveals the presence of capsules (pores) with a size of 5–15 nm; their existence is also unambiguously con firmed by the Xray reflectometry data. The depth of location of the found incapsulated nanoporous layer, reconstructed from the reflectome try data, is approximate, because it is impossible to take into account all structural features of sample N3EI in the model under consideration. The success ful fitting of XRR curves calls for the further develop ment of the model of scattering from internal porous
It was shown that highresolution Xray reflectom etry can successfully be used for diagnostics of the sur face layers of silicon substrates implanted by (2–5)keV He ions to doses of 1015–1017 cm–2. The experimental data were mathematically processed and the profiles of electron density distribution over the layer depth were reconstructed. It was shown that the layers have a complex structure, which includes an amorphized layer, an incapsulated porous layer, and an elastically strained damaged layer. It was established that an internal porous layer, containing pores 5–20 nm in diameter, is formed in the surface region at implanta tion doses of 1016 cm–2 and higher. The formation of pores leads to the occurrence of additional diffuse scattering at angles exceeding the critical one. This diffuse scattering can be used both for the fast moni toring of the formation of porous sublayer and for its characterization. The pore formation leads to the compacting of boundaries between the amorphized and porous layers. The layer model and the parameters of the sublayers depend on the implantation parame ters and may differ significantly. Therefore, their com plete description calls for complex methods. The layer parameters obtained based on Xray data are in good agreement with the calculated distribution of implantedhelium concentration, and the presence of pores and their size are consistent with the transmis sion electron microscopy (TEM) data. A more detailed diagnostics of the structural parameters of pores in buried porous layers can be performed using highpower Xray sources. ACKNOWLEDGMENTS We are grateful to M.A. Chuev for his interest in this study and for supplying the program for fitting Xray reflectometry curves and to A.L. Vasil’ev for his help in the electron microscopy analysis. This study was supported in part by the Russian Foundation for Basic Research, project no. 1207 00745a. REFERENCES 1. M. Nastasi and J. W. Mayer, Ion Implantation and Syn thesis of Materials (Springer, New York, 2006). 2. S. B. Felch, Z. Fang, B. W. Woo, et al., Surf. Coat. Technol. 156, 22 (2002). 3. V. A. Karavanskii, A. A. Lomov, E. V. Rakova, et al., Poverkhnost, No. 12, 32 (1999). 4. A. A. Lomov, V. A. Karavanskii, A. L. Vasil’ev, et al., Crystallogr. Rep. 53 (5), 742 (2008).
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XRAY REFLECTOMETRY OF THE SPECIFIC FEATURES OF STRUCTURAL DISTORTIONS 5. P. R. Desautels, M. P. Bradley, J. T. Steenkamp, and J. Mantyka, Phys. Status Solidi A 206, 985 (2009). 6. A. V. Myakon’kikh, A. E. Rogozhin, K. V. Rudenko, and V. F. Lukichev, Mikroelektronika 42 (4), 1 (2013). 7. D. Alquier, F. Roqueta, L. Ventura, et al., Jpn. J. Appl. Phys. A 41 (6), 3625 (2002). 8. http://sergey.gmca.aps.anl.gov/ 9. M. A. Chuev, I. A. Subbotin, E. M. Pashaev, et al., JETP Lett. 85 (1), 17 (2007). 10. A. M. Afanas’ev, M. A. Chuev, R. M. Imamov, et al., Crystallogr. Rep. 42 (3), 467 (1997). 11. Sh. Sh. Ibragimov and V. F. Reutov, USSR Inventor’s Certificate No. 1 282 757 A1, 1983. 12. R. Siegele, G. C. Weatherly, H. K. Haugen, et al., Appl. Phys. Lett. 66, 1319 (1995).
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13. V. Raineri, S. Coffa, E. Szilagyi, et al., Phys. Rev. В. 61, 937 (2000). 14. A. A. Lomov, V. A. Karavanskii, R. M. Imamov, et al., Crystallogr. Rep. 47 (6), 1051 (2002). 15. A. G. Sutyrin, V. A. Bushuev, and A. A. Lomov, Izv. Ross. Akad. Nauk, Ser. Fiz. 68 (4), 545 (2004). 16. http://www.srim.org/ 17. J. M. Chesnokov, A. L. Vasiliev, V. F. Lukichev, and K. V. Rudenko, Proc of 18th Microscopy of SemiCon ducting Materials Conference (MSMXVIII), Oxford, UK, 7–11 April 2013, Vol. 1, p. 115.
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Translated by Yu. Sin’kov
