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Strain-controlled boron and nitrogen doping of amorphous carbon layers for hard mask applications
Diamond & Related Materials 69 (2016) 102–107
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Strain-controlled boron and nitrogen doping of amorphous carbon layers for hard mask applications TaeWan Kim a,1, Dongbin Kim a,g,1, Yong-Sung Kim b,e, Sang Hyun Park c, Sung Kyu Lim c, Keun Oh. Park f, Taesung Kim g, Sang-Woo Kang a,d,⁎ a
Center for Vacuum, Korea Research Institute of Standards and Science, Daejeon 305-340, Republic of Korea Materials Genome Center, Korea Research Institute of Standards and Science, Daejeon 305-340, Republic of Korea c National Nanofab Center, Korea Advanced Institute of Science and Technology, Daejeon 305-338, Republic of Korea d Department of Advanced Device Technology, University of Science and Technology, Daejeon 305-350, Republic of Korea e Department of Nano Science, University of Science and Technology, Daejeon 305-350, Republic of Korea f TES Company Limited, Yongin, Gyeonggi 237-436, Republic of Korea g School of Mechanical Engineering, Sungkyunkwan University, Suwon, Gyeonggi 440-746, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 8 April 2016 Received in revised form 22 July 2016 Accepted 13 August 2016 Available online 14 August 2016 Keywords: Amorphous carbon layer Strain-control Dopant Diborane Ammonia
a b s t r a c t Nitrogen- and boron-doped amorphous carbon layers (ACLs) were grown by plasma- enhanced chemical vapor deposition (PECVD) on a Si substrate and characterized by Raman and X-ray photoelectron spectroscopy (XPS) techniques. Increasing doping levels resulted in a shift in the Raman G-peak of the doped ALCs, indicating a change in bond lengths upon doping. The XPS N1s and B1s spectra revealed the presence of different types of N-related (involving pyridinic, pyrrolinic, and graphitic N) and B-related (corresponding to BC2O and B\\C species) bonds in the N- and B-doped ACLs, with N and B doping levels ranging from 2.03 to 3.94 at.% and from 1.44 to 10.4 at.%, respectively. These results suggest that the dry etch resistance of the present ACLs was enhanced by B doping and negatively affected by N doping. Density functional theory calculations highlighted the strengthening of C\\C bonds induced by B doping and their corresponding weakening caused by N doping as possible explanations for the effects of doping on the dry etching characteristics of the ACLs. © 2016 Published by Elsevier B.V.
1. Introduction Due to the rapidly growing demand for advanced mobile storage devices, including systems such as solid state drives (SSD), recent years have witnessed a dramatic increase in the density of NAND flash memories in state-of-the-art technologies. A three-dimensional (3D) type of NAND flash memory (vertical-channel stacked array) has been recently introduced with the aim to reduce the price per bit in SSD devices through scaling down and increasing degree of integration, and to overcome several physical limitations of the two-dimensional (2D) lateral scaling lithography process, such as short channel effects, cell to cell optical interferences, and high coupling ratio [1]. Lithography and dry etching processes used for fabricating the 3D NAND structure require a hard mask including high aspect ratio resist patterns [2]. A poor dry etching resistance and a thin photoresist (PR) thickness represent challenging issues for lithography processes using a conventional Si3N4 hard mask [3]. Previous studies indicate that multilayer resist structures ⁎ Corresponding author at: Center for Vacuum, Korea Research Institute of Standards and Science, Daejeon 305-340, Republic of Korea. E-mail address: [email protected] (S.-W. Kang). 1 These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.diamond.2016.08.002 0925-9635/© 2016 Published by Elsevier B.V.
(MLR) can overcome these issues by utilizing SiON (or SiOx) and amorphous carbon as hard mask and Si3N4 (or SiO2) as underlayer [3]. Amorphous carbon layers (ACLs) are promising hard mask materials, due to favorable properties such as high etch selectivity (10:1 oxide to amorphous carbon etch rate), high transparency, easy elimination by oxygen plasma, and high durability in plasma asher [4–6]. No previous systematic studies have been reported on the impact of B and N doping on the chemical structure and dry etching properties of ACLs. The present investigation aims at elucidating the effect of boron and nitrogen doping on the etch rate of ACLs. We report the chemical and dry etch characteristics of B- and N-doped ACLs grown by plasmaenhanced chemical vapor deposition (PECVD). The alternated chemical bond structure (i.e. bond length changes) of the ACLs achieved through additional doping and its effects on the dry etching properties of the material are discussed. 2. Methods ACLs (500 nm-thick) were grown on a p-type Si substrate via PECVD. Acetylene (C2H2) was used as carbon precursor, while B2H6 and NH3 were used as p- and n-doping sources. Bulk ACLs were grown under a reactor pressure of 5 Torr, a temperature of 400 °C, and a radiofrequency
T. Kim et al. / Diamond & Related Materials 69 (2016) 102–107
(RF) power of 700 W. Etching of the bulk ACLs was carried out using a reactive ion etcher (Exelan HPT, LAM Research) with the following process conditions: chamber pressure of 70 mTorr, RF power of 250 W, C4F8 flow of 10 sccm, CH2F2 flow of 12 sccm, Ar flow of 400 sccm, and O2 flow of 14 sccm. Raman spectroscopy measurements were performed at room temperature using a DXT Raman spectrometer (Thermo Scientific). XPS (SES-100, VG-SCIENTA) measurements were conducted using a nonmonochromatic aluminum Kα source under ultrahigh vacuum conditions (pressure b 10−8 Torr). The etch rates of the ACLs were measured using a SFX100 spectroscopic ellipsometer (KLA-Tencor). Density functional theory (DFT) calculations were performed using the Vienna ab initio simulation (VASP) package [7,8]. Projector augmented wave (PAW) pseudopotentials [9] and the local density approximation (LDA) [10] were employed to describe the exchange-correlation functional. A kinetic energy cutoff of 300 eV was used for the plane-wave basis set expansion. The amorphous structures were generated by melt-and-quench molecular dynamics simulations within the Nosé canonical ensemble. A crystal cubic diamond structure with 64 atoms was melted at 5000 K for 3 ps and then quenched down to 0 K with a quenching rate of 1667 K/ps. The resulting amorphous structures were then optimized by static DFT calculations until the Hellmann– Feynman forces were b0.02 eV/Å. A single (0.25, 0.25, 0.25) k-point in the cubic Brillouin zone (BZ) was used for the BZ integration [18]. The total energy difference between the structures optimized using the single k-point and a 2 × 2 × 2 Monkhorst–Pack mesh was only a few meV. The experimental lattice constant of 5.431 Å was used in the calculations, and the atomic density was assumed to be the same for all the ACLs.
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3. Results and discussion We first investigated the atomic structure of ACLs with increasing doping levels, based on the DFT calculations. B- and N-doping levels of 1.6, 3.1, 4.7, 6.3, and 7.8 at.% were considered. Fig. 1(a) shows the amorphous structures of the undoped ACL obtained from the calculations, whereas Fig. 1(b) and (c) show the structure of B- and N-doped ACLs, respectively, obtained for a representative doping level of 7.8 at.%. The atomic radial pair distribution functions (RPDFs) of the amorphous structures are plotted in Fig. 1(d) for the B-doped and Fig. 1(e) for the N-doped cases. The definition of the pair distribution function (g(r)) is g ðrÞ ¼ ρ1 h∑ δðr−ri Þi, where ρ is the atomic density, ri is the atomic posii≠0
tions and i is the atom index. The RPDF G(R) is then G(R) =∫R0g(r)dr with the angular integral. As the doping level is increased, the RPDFs are shifted to shorter bond lengths for the B-doped samples and to longer bond lengths for the N-doped ones. These tendencies are seen more clearly in the accumulated RPDFs (A-RPDFs) shown in Fig. 1(f) and (g). The definition of the A-RPDF G(R) is G(R) = ∫R0g(r)dr, that is the number of atoms in the sphere of radius R. As the radial distance from an atomic center increases to about 1.8 Å, the A-RPDFs converge to four, indicating fourfold coordination of the C, B, and N atoms in the ACLs. The alteration of the bond lengths introduced by the doping is evident in the region between 1.45 and 1.60 Å: shorter and longer bond lengths result from B- and N-doping, respectively. The cause of the bond length changes observed upon B- and N-doping of the ACLs must be investigated. Fig. 1(h) and (i) show the separate
Fig. 1. (a–c): Atomic structures of (a) undoped, (b) 7.8 at.% B-doped, and (c) 7.8 at.% N-doped amorphous carbon layers (ACLs), generated by density functional theory (DFT)-based meltand-quench molecular dynamics simulations. (d, e): Calculated radial pair distribution functions (RPDFs) for all atomic species including the dopants for (d) B-doped and (e) N-doped ACLs. (f, g): Accumulated RPDFs (A-RPDFs) for all atomic species including the dopants for (f) B-doped and (g) N-doped ACLs. The arrows in panels (d)–(g) indicate increasing doping levels. (h, i): RPDFs corresponding to C and dopant species calculated for (h) B-doped and (i) N-doped ACLs. (j, k): Schematic illustration of strain surrounding the B (j) and N (k) dopant atoms in ACL.
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RPDFs of C and dopant atoms, for the representative 7.8 at.% doping level. The C\\B and C\\N bond lengths lie in the 1.4–1.6 and 1.3–1.5 Å ranges, respectively. On average, the C\\B bond lengths in the amorphous structure are slightly longer than the C\\C bond lengths, while the C\\N bond lengths are shorter (compare the C\\C and C\\B or C\\N RPDFs in Fig. 1(h) and (i)). However, the observation of C-dopant bond lengths different from C\\C bond lengths in the host as a result of the doping appears at odds with the atomic A-RPDF results shown in Fig. 1(f) and (g). Although the average C\\B bond lengths in B-doped ACL are longer than the C\\C bond lengths, bond shortening is observed in the atomic A-RPDF plotted in Fig. 1(f). Similarly, the C\\N bond lengths in the N-doped ACL are on average shorter than the C\\C bond lengths, but the atomic A-RPDF in Fig. 1(g) show the opposite trend for these bond lengths. These discrepancies can be understood through the schemes shown in Fig. 1(j) and (k). In the doped ACLs, attraction or repulsion between the dopant and the nearest-neighbor C atoms result in shorter N\\C or longer B\\C bond lengths, respectively. As a result, the surrounding regions undergo tensile or compressive strain. The ACLs exhibit several Raman peaks between 1200 and 1800 cm−1, including the defect or disorder peak (D-peak) at ~1348 cm−1 and the E2g mode G-peak at ~1590 cm−1. The Raman Gpeak of N-doped ACL shifts to larger wavenumbers as the NH3/C2H2 ratio increases from 0 to 0.36, as shown in Fig. 2(a, b), possibly due to the shorter B\\C bond lengths relative to the C\\C bonds, the smaller sp2 domain size, and/or C_N band overlap [11]. The Raman D-peaks of the B-doped ACL exhibit a different red-shift behavior as the B2H6/ C2H2 ratio increases from 0 to 16, which may be attributed to the longer C\\B bond lengths compared with the C\\C ones, as discussed in connection with the DFT results (Fig. 3(a, b)). The ratio of the D and G peak intensities (ID/IG) is typically used to quantify the carbon disorder (i.e., the relative proportion of sp2 and sp3 content) [12–14]. As the lightly N-doped level increases in ACL, ID/IG values are only weakly influenced (0.76 to 0.80), which is different behavior to that reported for highly N-doped ACL [15,16]. By contrast, Increasing B doping concentration in the ACLs results in the ID/IG ratio decreasing from 0.76 to 0.59, indicating that B doping leads to the elimination of defect sites in the ACLs and to an increase in the fraction of sp2 carbon, which is in close agreement to those simulated by DFT calculations [15]. XPS measurements were used to determine the elemental composition and characterize the chemical states in B- and N-doped ACLs. As shown in Figs. 4 and 5, a full–range XPS spectra of N-doped ACL exhibits B1s, C1s, N1s, and O1s spectra with corresponding main peaks centered at ~ 191.2 eV, ~ 284.5 eV, ~ 399 eV, and ~ 532.5 eV, respectively. The
dominant C1s peaks at 284.5 eV were assigned to sp2-hybridized graphite-like carbon atoms in C_C bonds. The N1 s peak in the 399–401 eV range is attributed to pyridinic, pyrrolic, and graphitic N [13,16]. B\\C bonds and the oxide-related (BC2O) species give rise to the B1s peak in the range of 189–192 eV [12]. An enhancement of the peak intensity in the N1s and B1s spectra is observed for N- and B-doped ACLs prepared with higher NH3 and B2H6 flow, and it results from a higher N and B content, respectively. The atomic percentage of N was estimated to be 2.03, 2.91, 3.94, and 3.9% for NH3/C2H2 ratios of 0.04, 0.16, 0.24, and 0.36, respectively, while that of B was calculated to be 1.44, 1.78, 3.62, 5.62, 7.88, and 10.4% for B2H6/C2H2 ratios of 1.6, 4, 8, 12, and 16, respectively. The O1s peaks at 532.5 eV could derive from oxygen contamination at the surface and/or C\\OH groups [13,17]. The effects of B and N doping on the etch rate were then examined. A previous study has demonstrated that a hard mask employing the Bdoped ACL leads to higher durability during the etching process employed for nanoscale fabrication, relative to the durability of undoped ACL [6]. We observed a significant reduction in the dry etching rate of B-doped ACL films with increasing B2H6/C2H2 ratio to values above 8, as shown in Fig. 6(b). By contrast, N-doped ACLs exhibit a slightly increase in dry etching rate (Fig. 6(a)). These different behaviors can be explained through the DFT results. Longer C\\B bonds lead to shorter and thus stronger C\\C bonds and high local-density of carbon in ACL, which are more resistant to the etching process. On the other hand, short C\\N bonds result in longer C\\C bonds, which can be more easily etched away. The etch rate of ACL films were found to be not significantly difference at the various oxygen employed here. 4. Conclusions In summary, we employed PECVD to synthesize B- and N-doped ACLs with N and B dopant concentrations of 2.03–3.9 at.% and 1.44– 10.4 at.%, respectively, for hard mask applications. DFT calculations demonstrate that N and B doping yield tensile and compressive strain, respectively, owing to a change in C\\C bond lengths. We have investigated the influence of B and N doping on the dry etching properties of ACLs. Boron doping leads to compressive strain, which correlates with longer bond lengths and hence significantly reduces the dry etching rate. On the contrary, N-doped ACLs exhibit a higher dry etching rate, compared with the undoped film. The present experimental results are consistent with the predictions of ab initio calculations. The potential reduction of the dry etching rate of ACLs that can be achieved through higher B doping levels makes these systems promising candidates for hard mask applications.
Fig. 2. (a) Raman spectra and (b) difference between D and G peak position of N-doped amorphous carbon layers (ACLs) vs. NH3/C2H2 ratio.
T. Kim et al. / Diamond & Related Materials 69 (2016) 102–107
Fig. 3. (a) Raman spectra and (b) difference between D and G peak of B-doped amorphous carbon layers (ACLs) vs. B2H6/C2H2 ratio.
Fig. 4. (a) C1s, (b) N1s, (c) O1s, and (d) full-range X-ray photoelectron spectroscopy (XPS) spectra of N-doped amorphous carbon layers (ACLs).
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Fig. 5. (a) C1s, (b) B1s, (c) O1s, and (d) full-range X-ray photoelectron spectroscopy (XPS) spectra of B-doped amorphous carbon layers (ACLs).
Fig. 6. Dry etch rate of (a) N-doped amorphous carbon layers (ACLs) vs. NH3/C2H2 ratio and of (b) B-doped ACLs vs. B2H6/C2H2 ratio.
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Prime novelty statement The key finding of our work is the experimental observation that the dry etch resistance of the ACLs is enhanced by boron doping and negatively affected by nitrogen doping. References [1] S.H. Park, Y. Kim, W. Kim, J.Y. Seo, B.G. Park, Solid State Electron. 78 (2012) 34–38. [2] S. Pauliac-Vaujour, P. Brianceau, C. Comboroure, O. Faynot, Microelectron. Eng. 85 (2008) 800–804. [3] B.S. Kwon, J.S. Kim, N.E. Lee, J.W. Shon, J. Electrochem. Soc. 157 (2010) D135–D141. [4] J.K. Kim, S.I. Cho, N.G. Kim, M.S. Jhon, K.S. Min, C.K. Kim, G.Y. Yeom, J. Vac. Sci. Technol. A 31 (2013) 021301.
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[5] S. Lee, J. Won, J. Choi, J. Park, Y. Jee, H. Lee, D. Byun, Thin Solid Films 519 (2011) 6683–6687. [6] Z. Yin, G.S. Sandhu, US Patent 6,939,794 B2, 2005. [7] G. Kresse, J. Furthmüller, Phys. Rev. B 54 (1996) 11169–11186. [8] G. Kresse, J. Joubert, Phys. Rev. B 59 (1999) 1758–1775. [9] P. Blöchl, Phys. Rev. B 50 (1994) 17953–17978. [10] D.M. Ceperley, B.J. Alder, Phys. Rev. Lett. 45 (1980) 566–569. [11] J.H. Kaufman, S. Metin, D.D. Saperstein, Phys. Rev. B 39 (1989) 53–60. [12] C.H. Choi, S.H. Park, S.I. Woo, ACS Nano 6 (2012) 7084–7091. [13] W. Guo, X. Li, J. Xu, H.K. Liu, J. Ma, S.X. Dou, Electrochim. Acta 188 (2016) 414–420. [14] R.U.R. Sagar, X. Zhang, C. Xiong, Y. Yu, Carbon 76 (2014) 64–70. [15] J.W. Suk, S. Murali, J. An, R.S. Ruoff, Carbon 50 (2012) 2220–2225. [16] W. Guo, X. Li, J. Xu, H.K. Liu, J. Ma, S.X. Dou, Electrochim. Acta 188 (2016) 414–420. [17] J. Li, Z. Ren, Y. Zhou, X. Wu, X. Xu, M. Qi, W. Li, J. Bai, L. Wang, Carbon 47 (2013) 330–336. [18] D.J. Chadi, M.L. Cohen, Phys. Rev. B 8 (1973) 5747–5753.
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Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Strain-controlled boron and nitrogen doping of amorphous carbon layers for hard mask applications TaeWan Kim a,1, Dongbin Kim a,g,1, Yong-Sung Kim b,e, Sang Hyun Park c, Sung Kyu Lim c, Keun Oh. Park f, Taesung Kim g, Sang-Woo Kang a,d,⁎ a
Center for Vacuum, Korea Research Institute of Standards and Science, Daejeon 305-340, Republic of Korea Materials Genome Center, Korea Research Institute of Standards and Science, Daejeon 305-340, Republic of Korea c National Nanofab Center, Korea Advanced Institute of Science and Technology, Daejeon 305-338, Republic of Korea d Department of Advanced Device Technology, University of Science and Technology, Daejeon 305-350, Republic of Korea e Department of Nano Science, University of Science and Technology, Daejeon 305-350, Republic of Korea f TES Company Limited, Yongin, Gyeonggi 237-436, Republic of Korea g School of Mechanical Engineering, Sungkyunkwan University, Suwon, Gyeonggi 440-746, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 8 April 2016 Received in revised form 22 July 2016 Accepted 13 August 2016 Available online 14 August 2016 Keywords: Amorphous carbon layer Strain-control Dopant Diborane Ammonia
a b s t r a c t Nitrogen- and boron-doped amorphous carbon layers (ACLs) were grown by plasma- enhanced chemical vapor deposition (PECVD) on a Si substrate and characterized by Raman and X-ray photoelectron spectroscopy (XPS) techniques. Increasing doping levels resulted in a shift in the Raman G-peak of the doped ALCs, indicating a change in bond lengths upon doping. The XPS N1s and B1s spectra revealed the presence of different types of N-related (involving pyridinic, pyrrolinic, and graphitic N) and B-related (corresponding to BC2O and B\\C species) bonds in the N- and B-doped ACLs, with N and B doping levels ranging from 2.03 to 3.94 at.% and from 1.44 to 10.4 at.%, respectively. These results suggest that the dry etch resistance of the present ACLs was enhanced by B doping and negatively affected by N doping. Density functional theory calculations highlighted the strengthening of C\\C bonds induced by B doping and their corresponding weakening caused by N doping as possible explanations for the effects of doping on the dry etching characteristics of the ACLs. © 2016 Published by Elsevier B.V.
1. Introduction Due to the rapidly growing demand for advanced mobile storage devices, including systems such as solid state drives (SSD), recent years have witnessed a dramatic increase in the density of NAND flash memories in state-of-the-art technologies. A three-dimensional (3D) type of NAND flash memory (vertical-channel stacked array) has been recently introduced with the aim to reduce the price per bit in SSD devices through scaling down and increasing degree of integration, and to overcome several physical limitations of the two-dimensional (2D) lateral scaling lithography process, such as short channel effects, cell to cell optical interferences, and high coupling ratio [1]. Lithography and dry etching processes used for fabricating the 3D NAND structure require a hard mask including high aspect ratio resist patterns [2]. A poor dry etching resistance and a thin photoresist (PR) thickness represent challenging issues for lithography processes using a conventional Si3N4 hard mask [3]. Previous studies indicate that multilayer resist structures ⁎ Corresponding author at: Center for Vacuum, Korea Research Institute of Standards and Science, Daejeon 305-340, Republic of Korea. E-mail address: [email protected] (S.-W. Kang). 1 These authors contributed equally to this work.
http://dx.doi.org/10.1016/j.diamond.2016.08.002 0925-9635/© 2016 Published by Elsevier B.V.
(MLR) can overcome these issues by utilizing SiON (or SiOx) and amorphous carbon as hard mask and Si3N4 (or SiO2) as underlayer [3]. Amorphous carbon layers (ACLs) are promising hard mask materials, due to favorable properties such as high etch selectivity (10:1 oxide to amorphous carbon etch rate), high transparency, easy elimination by oxygen plasma, and high durability in plasma asher [4–6]. No previous systematic studies have been reported on the impact of B and N doping on the chemical structure and dry etching properties of ACLs. The present investigation aims at elucidating the effect of boron and nitrogen doping on the etch rate of ACLs. We report the chemical and dry etch characteristics of B- and N-doped ACLs grown by plasmaenhanced chemical vapor deposition (PECVD). The alternated chemical bond structure (i.e. bond length changes) of the ACLs achieved through additional doping and its effects on the dry etching properties of the material are discussed. 2. Methods ACLs (500 nm-thick) were grown on a p-type Si substrate via PECVD. Acetylene (C2H2) was used as carbon precursor, while B2H6 and NH3 were used as p- and n-doping sources. Bulk ACLs were grown under a reactor pressure of 5 Torr, a temperature of 400 °C, and a radiofrequency
T. Kim et al. / Diamond & Related Materials 69 (2016) 102–107
(RF) power of 700 W. Etching of the bulk ACLs was carried out using a reactive ion etcher (Exelan HPT, LAM Research) with the following process conditions: chamber pressure of 70 mTorr, RF power of 250 W, C4F8 flow of 10 sccm, CH2F2 flow of 12 sccm, Ar flow of 400 sccm, and O2 flow of 14 sccm. Raman spectroscopy measurements were performed at room temperature using a DXT Raman spectrometer (Thermo Scientific). XPS (SES-100, VG-SCIENTA) measurements were conducted using a nonmonochromatic aluminum Kα source under ultrahigh vacuum conditions (pressure b 10−8 Torr). The etch rates of the ACLs were measured using a SFX100 spectroscopic ellipsometer (KLA-Tencor). Density functional theory (DFT) calculations were performed using the Vienna ab initio simulation (VASP) package [7,8]. Projector augmented wave (PAW) pseudopotentials [9] and the local density approximation (LDA) [10] were employed to describe the exchange-correlation functional. A kinetic energy cutoff of 300 eV was used for the plane-wave basis set expansion. The amorphous structures were generated by melt-and-quench molecular dynamics simulations within the Nosé canonical ensemble. A crystal cubic diamond structure with 64 atoms was melted at 5000 K for 3 ps and then quenched down to 0 K with a quenching rate of 1667 K/ps. The resulting amorphous structures were then optimized by static DFT calculations until the Hellmann– Feynman forces were b0.02 eV/Å. A single (0.25, 0.25, 0.25) k-point in the cubic Brillouin zone (BZ) was used for the BZ integration [18]. The total energy difference between the structures optimized using the single k-point and a 2 × 2 × 2 Monkhorst–Pack mesh was only a few meV. The experimental lattice constant of 5.431 Å was used in the calculations, and the atomic density was assumed to be the same for all the ACLs.
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3. Results and discussion We first investigated the atomic structure of ACLs with increasing doping levels, based on the DFT calculations. B- and N-doping levels of 1.6, 3.1, 4.7, 6.3, and 7.8 at.% were considered. Fig. 1(a) shows the amorphous structures of the undoped ACL obtained from the calculations, whereas Fig. 1(b) and (c) show the structure of B- and N-doped ACLs, respectively, obtained for a representative doping level of 7.8 at.%. The atomic radial pair distribution functions (RPDFs) of the amorphous structures are plotted in Fig. 1(d) for the B-doped and Fig. 1(e) for the N-doped cases. The definition of the pair distribution function (g(r)) is g ðrÞ ¼ ρ1 h∑ δðr−ri Þi, where ρ is the atomic density, ri is the atomic posii≠0
tions and i is the atom index. The RPDF G(R) is then G(R) =∫R0g(r)dr with the angular integral. As the doping level is increased, the RPDFs are shifted to shorter bond lengths for the B-doped samples and to longer bond lengths for the N-doped ones. These tendencies are seen more clearly in the accumulated RPDFs (A-RPDFs) shown in Fig. 1(f) and (g). The definition of the A-RPDF G(R) is G(R) = ∫R0g(r)dr, that is the number of atoms in the sphere of radius R. As the radial distance from an atomic center increases to about 1.8 Å, the A-RPDFs converge to four, indicating fourfold coordination of the C, B, and N atoms in the ACLs. The alteration of the bond lengths introduced by the doping is evident in the region between 1.45 and 1.60 Å: shorter and longer bond lengths result from B- and N-doping, respectively. The cause of the bond length changes observed upon B- and N-doping of the ACLs must be investigated. Fig. 1(h) and (i) show the separate
Fig. 1. (a–c): Atomic structures of (a) undoped, (b) 7.8 at.% B-doped, and (c) 7.8 at.% N-doped amorphous carbon layers (ACLs), generated by density functional theory (DFT)-based meltand-quench molecular dynamics simulations. (d, e): Calculated radial pair distribution functions (RPDFs) for all atomic species including the dopants for (d) B-doped and (e) N-doped ACLs. (f, g): Accumulated RPDFs (A-RPDFs) for all atomic species including the dopants for (f) B-doped and (g) N-doped ACLs. The arrows in panels (d)–(g) indicate increasing doping levels. (h, i): RPDFs corresponding to C and dopant species calculated for (h) B-doped and (i) N-doped ACLs. (j, k): Schematic illustration of strain surrounding the B (j) and N (k) dopant atoms in ACL.
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RPDFs of C and dopant atoms, for the representative 7.8 at.% doping level. The C\\B and C\\N bond lengths lie in the 1.4–1.6 and 1.3–1.5 Å ranges, respectively. On average, the C\\B bond lengths in the amorphous structure are slightly longer than the C\\C bond lengths, while the C\\N bond lengths are shorter (compare the C\\C and C\\B or C\\N RPDFs in Fig. 1(h) and (i)). However, the observation of C-dopant bond lengths different from C\\C bond lengths in the host as a result of the doping appears at odds with the atomic A-RPDF results shown in Fig. 1(f) and (g). Although the average C\\B bond lengths in B-doped ACL are longer than the C\\C bond lengths, bond shortening is observed in the atomic A-RPDF plotted in Fig. 1(f). Similarly, the C\\N bond lengths in the N-doped ACL are on average shorter than the C\\C bond lengths, but the atomic A-RPDF in Fig. 1(g) show the opposite trend for these bond lengths. These discrepancies can be understood through the schemes shown in Fig. 1(j) and (k). In the doped ACLs, attraction or repulsion between the dopant and the nearest-neighbor C atoms result in shorter N\\C or longer B\\C bond lengths, respectively. As a result, the surrounding regions undergo tensile or compressive strain. The ACLs exhibit several Raman peaks between 1200 and 1800 cm−1, including the defect or disorder peak (D-peak) at ~1348 cm−1 and the E2g mode G-peak at ~1590 cm−1. The Raman Gpeak of N-doped ACL shifts to larger wavenumbers as the NH3/C2H2 ratio increases from 0 to 0.36, as shown in Fig. 2(a, b), possibly due to the shorter B\\C bond lengths relative to the C\\C bonds, the smaller sp2 domain size, and/or C_N band overlap [11]. The Raman D-peaks of the B-doped ACL exhibit a different red-shift behavior as the B2H6/ C2H2 ratio increases from 0 to 16, which may be attributed to the longer C\\B bond lengths compared with the C\\C ones, as discussed in connection with the DFT results (Fig. 3(a, b)). The ratio of the D and G peak intensities (ID/IG) is typically used to quantify the carbon disorder (i.e., the relative proportion of sp2 and sp3 content) [12–14]. As the lightly N-doped level increases in ACL, ID/IG values are only weakly influenced (0.76 to 0.80), which is different behavior to that reported for highly N-doped ACL [15,16]. By contrast, Increasing B doping concentration in the ACLs results in the ID/IG ratio decreasing from 0.76 to 0.59, indicating that B doping leads to the elimination of defect sites in the ACLs and to an increase in the fraction of sp2 carbon, which is in close agreement to those simulated by DFT calculations [15]. XPS measurements were used to determine the elemental composition and characterize the chemical states in B- and N-doped ACLs. As shown in Figs. 4 and 5, a full–range XPS spectra of N-doped ACL exhibits B1s, C1s, N1s, and O1s spectra with corresponding main peaks centered at ~ 191.2 eV, ~ 284.5 eV, ~ 399 eV, and ~ 532.5 eV, respectively. The
dominant C1s peaks at 284.5 eV were assigned to sp2-hybridized graphite-like carbon atoms in C_C bonds. The N1 s peak in the 399–401 eV range is attributed to pyridinic, pyrrolic, and graphitic N [13,16]. B\\C bonds and the oxide-related (BC2O) species give rise to the B1s peak in the range of 189–192 eV [12]. An enhancement of the peak intensity in the N1s and B1s spectra is observed for N- and B-doped ACLs prepared with higher NH3 and B2H6 flow, and it results from a higher N and B content, respectively. The atomic percentage of N was estimated to be 2.03, 2.91, 3.94, and 3.9% for NH3/C2H2 ratios of 0.04, 0.16, 0.24, and 0.36, respectively, while that of B was calculated to be 1.44, 1.78, 3.62, 5.62, 7.88, and 10.4% for B2H6/C2H2 ratios of 1.6, 4, 8, 12, and 16, respectively. The O1s peaks at 532.5 eV could derive from oxygen contamination at the surface and/or C\\OH groups [13,17]. The effects of B and N doping on the etch rate were then examined. A previous study has demonstrated that a hard mask employing the Bdoped ACL leads to higher durability during the etching process employed for nanoscale fabrication, relative to the durability of undoped ACL [6]. We observed a significant reduction in the dry etching rate of B-doped ACL films with increasing B2H6/C2H2 ratio to values above 8, as shown in Fig. 6(b). By contrast, N-doped ACLs exhibit a slightly increase in dry etching rate (Fig. 6(a)). These different behaviors can be explained through the DFT results. Longer C\\B bonds lead to shorter and thus stronger C\\C bonds and high local-density of carbon in ACL, which are more resistant to the etching process. On the other hand, short C\\N bonds result in longer C\\C bonds, which can be more easily etched away. The etch rate of ACL films were found to be not significantly difference at the various oxygen employed here. 4. Conclusions In summary, we employed PECVD to synthesize B- and N-doped ACLs with N and B dopant concentrations of 2.03–3.9 at.% and 1.44– 10.4 at.%, respectively, for hard mask applications. DFT calculations demonstrate that N and B doping yield tensile and compressive strain, respectively, owing to a change in C\\C bond lengths. We have investigated the influence of B and N doping on the dry etching properties of ACLs. Boron doping leads to compressive strain, which correlates with longer bond lengths and hence significantly reduces the dry etching rate. On the contrary, N-doped ACLs exhibit a higher dry etching rate, compared with the undoped film. The present experimental results are consistent with the predictions of ab initio calculations. The potential reduction of the dry etching rate of ACLs that can be achieved through higher B doping levels makes these systems promising candidates for hard mask applications.
Fig. 2. (a) Raman spectra and (b) difference between D and G peak position of N-doped amorphous carbon layers (ACLs) vs. NH3/C2H2 ratio.
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Fig. 3. (a) Raman spectra and (b) difference between D and G peak of B-doped amorphous carbon layers (ACLs) vs. B2H6/C2H2 ratio.
Fig. 4. (a) C1s, (b) N1s, (c) O1s, and (d) full-range X-ray photoelectron spectroscopy (XPS) spectra of N-doped amorphous carbon layers (ACLs).
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Fig. 5. (a) C1s, (b) B1s, (c) O1s, and (d) full-range X-ray photoelectron spectroscopy (XPS) spectra of B-doped amorphous carbon layers (ACLs).
Fig. 6. Dry etch rate of (a) N-doped amorphous carbon layers (ACLs) vs. NH3/C2H2 ratio and of (b) B-doped ACLs vs. B2H6/C2H2 ratio.
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