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Seed-induced crystallization of polycrystalline germanium thin films at low temperature
Results in Physics 14 (2019) 102502
Contents lists available at ScienceDirect
Results in Physics journal homepage: www.elsevier.com/locate/rinp
Microarticle
Seed-induced crystallization of polycrystalline germanium thin films at low temperature Mingjun Jiang, Donghwan Ahn
T
⁎
School of Materials Science and Engineering, Kookmin University, Seoul 02707, Republic of Korea
A B S T R A C T
A recrystallization technology that is required for implementing Ge devices in the semiconductor back-end-of-line (BEOL) processing at temperatures below 450 °C is presented. A new seed-induced crystallization (SIC) is applied to the fabrication of polycrystalline Ge (poly-Ge) thin films at 400 °C. While the process successfully achieved the crystalline fraction greater than 90% in Ge, it overcomes the problem of metal contamination in traditional metal-induced crystallization (MIC) technology by reducing the metal contamination to the levels below the detection limit of X-ray photoelectron spectroscopy (XPS).
Introduction A number of studies have been conducted to overcome the limitations of metal line electrical interconnects in semiconductor chips; the integration of optical elements to form optical interconnect in BEOL plane of CMOS processing, for which Ge photonic devices are essential components, is considered to be one of promising approaches [1]. The greatest advantages of Ge are that it can be epitaxially grown on silicon wafers and that it is perfectly compatible with the existing silicon semiconductor process lines. However, due to the processing complexities and structural interference of the front-end formation of Ge photonic devices on the same Si wafer plane with CMOS transistors, the back-end formation of optical interconnect can be advantageous. Because it is not possible to grow single-crystal germanium thin films on top of inter-layer dielectric (ILD) oxide in the BEOL planes, it is necessary to find a method for recrystallizing amorphous Ge thin films at low temperature (< 450 °C), which would be a pivotal step for fabricating Ge photonic devices such as photodetectors on the BEOL plane. One of the most commonly used low-temperature crystallization method is metal-induced crystallization (MIC) [2]. Compared with solid phase crystallization (SPC) [3,4], MIC technology usually enables the crystallization at lower temperatures. The MIC costs less than other recrystallization technologies such as excimer laser annealing (ELA) and it exhibits good application prospects. However, metal pollution is a disadvantage that limits the applications of MIC or a similar metalinduced-lateral-crystallization (MILC, the one involving a lateral crystal growth extending from patterned MIC area) process [5–7]. Byun et al. improved the traditional MILC technology by developing a modified version called seed-induced lateral crystallization (SILC) [8], in which they demonstrated a better poly-Si thin film transistor (TFT)
⁎
performance, due to lower Ni precipitate contaminant in the film. In a consideration that the benefits of SIC/SILC process over MIC/MILC in silicon may extend to germanium cases in a similar manner, we experimented with SIC technology to form a poly-Ge thin film at 400 °C and have investigated how it compares to the MIC by verifying the degree of metal contamination. Methods The principle of SIC is that a small amount of metal seed is deposited on an amorphous film before annealing. The metal atoms migrate into the amorphous Ge (α-Ge) thin film, the Ge–Ge bond is weakened, and metal compounds are generated. During annealing process, as the metal-Ge bonds break and the Ge atoms are locally rearranged, metal atoms are squeezed out of the growing Ge crystals. Further, metal atoms diffuse rapidly, moving the crystallization front and forming a polycrystalline germanium film [9]. The crystallization of the α -Ge film occurs at a lower temperature than SPC. First, a 200 nm thick SiO2 layer was deposited by LPCVD on the silicon wafer, followed by the deposition of a 200 nm thick α -Ge layer through electron beam evaporation. A 10 nm thick Ni thin film was further sputtered on α -Ge; during the deposition, Ni atoms bombarded and penetrated the Ge film, thereby creating NiGe seeds. The sample was subjected to H2SO4 treatment at 70 °C for 20 s in order to eliminate the residual Ni film. Although the Ni film was almost completely eliminated, NiGe seeds remain on the surface. Finally, the sample was annealed in a furnace at 400 °C for 2 h under N2 gas flow. For comparison, two more samples were prepared by MIC and SPC with the same annealing time and temperature. Unlike the SIC process, the MIC process involves a direct anneal after Ni film deposition and then the
Corresponding author. E-mail address: [email protected] (D. Ahn).
https://doi.org/10.1016/j.rinp.2019.102502 Received 28 April 2019; Received in revised form 1 July 2019; Accepted 2 July 2019 Available online 04 July 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Results in Physics 14 (2019) 102502
M. Jiang and D. Ahn
absorption correction factor was assumed to be γ = 0.8 [12]. The Xc of the MIC poly-Ge thin film was determined to be 90.7%, and the Xc of the SIC poly-Ge thin film was 90.8% which demonstrates that both MIC and SIC can fabricate poly-Ge films at 400 °C with similar crystallinity. However, the grain size of SIC films is slightly larger than that of MIC films. The degree of metal contamination in the samples was also examined using XPS. The results in Fig. 2 show that 20% of the MIC polyGe thin film surface was Ni and that the Ni content decreased with depth. However, almost no Ni was detected on or in the SIC poly-Ge thin film. This clearly indicates that the Ni content in the SIC poly-Ge thin film was considerably insufficient for detection within the resolution range of the XPS. These results demonstrate that the SIC method for the recrystallization of α-Ge can be an excellent alternative to traditional MIC, because it effectively overcomes the drawback of metal contamination in the MIC process. We successfully developed SIC technology for Ge. SIC is a lowtemperature process with the advantages of simplicity in implementation, efficiency, and low cost. Although a film prepared by SIC has similar crystallinity to a film prepared by MIC at the same conditions, its grain size is superior to that of the MIC film. It also showed the potential to solve the problem of metal contamination, which have so far restricted wide application of the MIC technology. Thus, SIC may contribute in the field of BEOL fabrication of photodetectors for optical interconnection and in other areas requiring low-temperature processing, such as the fabrication of microelectronics on plastic substrates.
Fig. 1. Raman spectra of SIC poly-Ge, MIC poly-Ge, and SPC Ge films after 2 h long annealing at 400 °C.
Acknowledgements This research was supported by Leading Foreign Research Institute Recruitment Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (MSIP) (2013K1A4A3055679) and supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2018R1D1A1A02051101). References [1] Lee YHD, Lipson M. Back-end deposited silicon photonics for monolithic integration on CMOS. IEEE J Sel Top Quantum Electron 2013;19:8200207. https://doi.org/10. 1109/JSTQE.2012.2209865. [2] Uenuma M, Zheng B, Imazawa T, Horita M, Nishida T, Ishikawa Y, et al. Metalnanoparticle-induced crystallization of amorphous Ge film using ferritin. Appl Surf Sci 2012;258:3410–4. https://doi.org/10.1016/j.apsusc.2011.11.076. [3] Toko K, Nakao I, Sadoh T, Noguchi T, Miyao M. Electrical properties of poly-Ge on glass substrate grown by two-step solid-phase crystallization. Solid State Electron 2009;53:1159–64. https://doi.org/10.1016/j.sse.2009.08.002. [4] Toko K, Yoshimine R, Moto K, Suemasu T. High-hole mobility polycrystalline Ge on an insulator formed by controlling precursor atomic density for solid-phase crystallization. Sci Rep 2017;7:16981. https://doi.org/10.1038/s41598-017-17273-6. [5] Kanno H, Toko K, Sadoh T, Miyao M. Temperature dependent metal-induced lateral crystallization of amorphous SiGe on insulating substrate. Appl Phys Lett 2006;89:182120https://doi.org/10.1063/1.2374849. [6] Park J-H, Kapur P, Saraswat KC, Peng H. A very low temperature single crystal germanium growth process on insulating substrate using Ni-induced lateral crystallization for three-dimensional integrated circuits. Appl Phys Lett 2007;91:143107https://doi.org/10.1063/1.2793183. [7] Toko K, Kanno H, Kenjo A, Sadoh T, Asano T, Miyao M. Ni-imprint induced solidphase crystallization in Si1−xGex (x: 0–1) on insulator. Appl Phys Lett 2007;91:42111. https://doi.org/10.1063/1.2764447. [8] Byun CW, Son SW, Lee YW, Joo SK. High performance low temperature polycrystalline Si thin-film transistors fabricated by silicide seed-induced lateral crystallization. Electron Mater Lett 2012;8:251–8. https://doi.org/10.1007/s13391012-2079-x. [9] Tan Z, Heald SM, Rapposch M, Bouldin CE, Woicik JC. Gold-induced germanium crystallization. Phys Rev B 1992;46:9505–10. https://doi.org/10.1103/PhysRevB. 46.9505. [10] Campbell IH, Fauchet PM. The effects of microcrystal size and shape on the one phonon Raman spectra of crystalline semiconductors. Solid State Commun 1986;58:739–41. https://doi.org/10.1016/0038-1098(86)90513-2. [11] Okada T, Iwaki T, Kasahara H, Yamamoto K. Probing the Crystallinity of Evaporated Silicon Films by Raman Scattering. Jpn J Appl Phys 1985;24:161–5. https://doi.org/10.1143/jjap.24.161. [12] Peng S, Hu D, He D. Low-temperature preparation of polycrystalline germanium thin films by Al-induced crystallization. Appl Surf Sci 2012;258:6003–6. https:// doi.org/10.1016/j.apsusc.2012.02.080.
Fig. 2. Atomic % depth profile of (a) MIC poly-Ge and (b) SIC poly-Ge, as obtained by XPS.
removal of the Ni thin film. The SPC process involves a direct anneal without an Ni thin film after α -Ge deposition. Results and conclusions The Raman spectra of SIC poly-Ge, MIC poly-Ge, and SPC Ge are depicted in Fig. 1. There are clear Ge–Ge peaks in the cases of the SIC and MIC curves near 300 cm−1, demonstrating that poly-Ge thin films can be successfully fabricated by SIC and MIC. The SPC curve exhibits no peak at 300 cm−1, indicating that the SPC method is not capable of crystallizing amorphous germanium at 400 °C. The grain sizes of SIC poly-Ge and MIC poly-Ge were estimated by examining the full width at half maximum (FWHM) of the Ge–Ge peak near 300 cm−1. It is well known that the grain size is related to the FWHM: the smaller the FWHM, the larger the grain size of the poly-Ge film [10]. The FWHM of the SIC poly-Ge thin film was 8.22, and it was observed to be smaller than the FWHM of the MIC poly-Ge thin film that was prepared under similar conditions, which was 10.08. It indicates that the SIC method produced larger grains than those produced by the MIC method. To further analyze the properties of the Ge thin film, the crystallinity of the films was calculated using the following formula, which defines the crystalline fraction (Xc) using integral intensities as Ic/(Ic + γIa) [11]. Ic denotes the crystalline integral intensity near 300 cm−1, and Ia denotes the amorphous integral intensity near 270 cm−1. Further, the photo 2
Contents lists available at ScienceDirect
Results in Physics journal homepage: www.elsevier.com/locate/rinp
Microarticle
Seed-induced crystallization of polycrystalline germanium thin films at low temperature Mingjun Jiang, Donghwan Ahn
T
⁎
School of Materials Science and Engineering, Kookmin University, Seoul 02707, Republic of Korea
A B S T R A C T
A recrystallization technology that is required for implementing Ge devices in the semiconductor back-end-of-line (BEOL) processing at temperatures below 450 °C is presented. A new seed-induced crystallization (SIC) is applied to the fabrication of polycrystalline Ge (poly-Ge) thin films at 400 °C. While the process successfully achieved the crystalline fraction greater than 90% in Ge, it overcomes the problem of metal contamination in traditional metal-induced crystallization (MIC) technology by reducing the metal contamination to the levels below the detection limit of X-ray photoelectron spectroscopy (XPS).
Introduction A number of studies have been conducted to overcome the limitations of metal line electrical interconnects in semiconductor chips; the integration of optical elements to form optical interconnect in BEOL plane of CMOS processing, for which Ge photonic devices are essential components, is considered to be one of promising approaches [1]. The greatest advantages of Ge are that it can be epitaxially grown on silicon wafers and that it is perfectly compatible with the existing silicon semiconductor process lines. However, due to the processing complexities and structural interference of the front-end formation of Ge photonic devices on the same Si wafer plane with CMOS transistors, the back-end formation of optical interconnect can be advantageous. Because it is not possible to grow single-crystal germanium thin films on top of inter-layer dielectric (ILD) oxide in the BEOL planes, it is necessary to find a method for recrystallizing amorphous Ge thin films at low temperature (< 450 °C), which would be a pivotal step for fabricating Ge photonic devices such as photodetectors on the BEOL plane. One of the most commonly used low-temperature crystallization method is metal-induced crystallization (MIC) [2]. Compared with solid phase crystallization (SPC) [3,4], MIC technology usually enables the crystallization at lower temperatures. The MIC costs less than other recrystallization technologies such as excimer laser annealing (ELA) and it exhibits good application prospects. However, metal pollution is a disadvantage that limits the applications of MIC or a similar metalinduced-lateral-crystallization (MILC, the one involving a lateral crystal growth extending from patterned MIC area) process [5–7]. Byun et al. improved the traditional MILC technology by developing a modified version called seed-induced lateral crystallization (SILC) [8], in which they demonstrated a better poly-Si thin film transistor (TFT)
⁎
performance, due to lower Ni precipitate contaminant in the film. In a consideration that the benefits of SIC/SILC process over MIC/MILC in silicon may extend to germanium cases in a similar manner, we experimented with SIC technology to form a poly-Ge thin film at 400 °C and have investigated how it compares to the MIC by verifying the degree of metal contamination. Methods The principle of SIC is that a small amount of metal seed is deposited on an amorphous film before annealing. The metal atoms migrate into the amorphous Ge (α-Ge) thin film, the Ge–Ge bond is weakened, and metal compounds are generated. During annealing process, as the metal-Ge bonds break and the Ge atoms are locally rearranged, metal atoms are squeezed out of the growing Ge crystals. Further, metal atoms diffuse rapidly, moving the crystallization front and forming a polycrystalline germanium film [9]. The crystallization of the α -Ge film occurs at a lower temperature than SPC. First, a 200 nm thick SiO2 layer was deposited by LPCVD on the silicon wafer, followed by the deposition of a 200 nm thick α -Ge layer through electron beam evaporation. A 10 nm thick Ni thin film was further sputtered on α -Ge; during the deposition, Ni atoms bombarded and penetrated the Ge film, thereby creating NiGe seeds. The sample was subjected to H2SO4 treatment at 70 °C for 20 s in order to eliminate the residual Ni film. Although the Ni film was almost completely eliminated, NiGe seeds remain on the surface. Finally, the sample was annealed in a furnace at 400 °C for 2 h under N2 gas flow. For comparison, two more samples were prepared by MIC and SPC with the same annealing time and temperature. Unlike the SIC process, the MIC process involves a direct anneal after Ni film deposition and then the
Corresponding author. E-mail address: [email protected] (D. Ahn).
https://doi.org/10.1016/j.rinp.2019.102502 Received 28 April 2019; Received in revised form 1 July 2019; Accepted 2 July 2019 Available online 04 July 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Results in Physics 14 (2019) 102502
M. Jiang and D. Ahn
absorption correction factor was assumed to be γ = 0.8 [12]. The Xc of the MIC poly-Ge thin film was determined to be 90.7%, and the Xc of the SIC poly-Ge thin film was 90.8% which demonstrates that both MIC and SIC can fabricate poly-Ge films at 400 °C with similar crystallinity. However, the grain size of SIC films is slightly larger than that of MIC films. The degree of metal contamination in the samples was also examined using XPS. The results in Fig. 2 show that 20% of the MIC polyGe thin film surface was Ni and that the Ni content decreased with depth. However, almost no Ni was detected on or in the SIC poly-Ge thin film. This clearly indicates that the Ni content in the SIC poly-Ge thin film was considerably insufficient for detection within the resolution range of the XPS. These results demonstrate that the SIC method for the recrystallization of α-Ge can be an excellent alternative to traditional MIC, because it effectively overcomes the drawback of metal contamination in the MIC process. We successfully developed SIC technology for Ge. SIC is a lowtemperature process with the advantages of simplicity in implementation, efficiency, and low cost. Although a film prepared by SIC has similar crystallinity to a film prepared by MIC at the same conditions, its grain size is superior to that of the MIC film. It also showed the potential to solve the problem of metal contamination, which have so far restricted wide application of the MIC technology. Thus, SIC may contribute in the field of BEOL fabrication of photodetectors for optical interconnection and in other areas requiring low-temperature processing, such as the fabrication of microelectronics on plastic substrates.
Fig. 1. Raman spectra of SIC poly-Ge, MIC poly-Ge, and SPC Ge films after 2 h long annealing at 400 °C.
Acknowledgements This research was supported by Leading Foreign Research Institute Recruitment Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (MSIP) (2013K1A4A3055679) and supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2018R1D1A1A02051101). References [1] Lee YHD, Lipson M. Back-end deposited silicon photonics for monolithic integration on CMOS. IEEE J Sel Top Quantum Electron 2013;19:8200207. https://doi.org/10. 1109/JSTQE.2012.2209865. [2] Uenuma M, Zheng B, Imazawa T, Horita M, Nishida T, Ishikawa Y, et al. Metalnanoparticle-induced crystallization of amorphous Ge film using ferritin. Appl Surf Sci 2012;258:3410–4. https://doi.org/10.1016/j.apsusc.2011.11.076. [3] Toko K, Nakao I, Sadoh T, Noguchi T, Miyao M. Electrical properties of poly-Ge on glass substrate grown by two-step solid-phase crystallization. Solid State Electron 2009;53:1159–64. https://doi.org/10.1016/j.sse.2009.08.002. [4] Toko K, Yoshimine R, Moto K, Suemasu T. High-hole mobility polycrystalline Ge on an insulator formed by controlling precursor atomic density for solid-phase crystallization. Sci Rep 2017;7:16981. https://doi.org/10.1038/s41598-017-17273-6. [5] Kanno H, Toko K, Sadoh T, Miyao M. Temperature dependent metal-induced lateral crystallization of amorphous SiGe on insulating substrate. Appl Phys Lett 2006;89:182120https://doi.org/10.1063/1.2374849. [6] Park J-H, Kapur P, Saraswat KC, Peng H. A very low temperature single crystal germanium growth process on insulating substrate using Ni-induced lateral crystallization for three-dimensional integrated circuits. Appl Phys Lett 2007;91:143107https://doi.org/10.1063/1.2793183. [7] Toko K, Kanno H, Kenjo A, Sadoh T, Asano T, Miyao M. Ni-imprint induced solidphase crystallization in Si1−xGex (x: 0–1) on insulator. Appl Phys Lett 2007;91:42111. https://doi.org/10.1063/1.2764447. [8] Byun CW, Son SW, Lee YW, Joo SK. High performance low temperature polycrystalline Si thin-film transistors fabricated by silicide seed-induced lateral crystallization. Electron Mater Lett 2012;8:251–8. https://doi.org/10.1007/s13391012-2079-x. [9] Tan Z, Heald SM, Rapposch M, Bouldin CE, Woicik JC. Gold-induced germanium crystallization. Phys Rev B 1992;46:9505–10. https://doi.org/10.1103/PhysRevB. 46.9505. [10] Campbell IH, Fauchet PM. The effects of microcrystal size and shape on the one phonon Raman spectra of crystalline semiconductors. Solid State Commun 1986;58:739–41. https://doi.org/10.1016/0038-1098(86)90513-2. [11] Okada T, Iwaki T, Kasahara H, Yamamoto K. Probing the Crystallinity of Evaporated Silicon Films by Raman Scattering. Jpn J Appl Phys 1985;24:161–5. https://doi.org/10.1143/jjap.24.161. [12] Peng S, Hu D, He D. Low-temperature preparation of polycrystalline germanium thin films by Al-induced crystallization. Appl Surf Sci 2012;258:6003–6. https:// doi.org/10.1016/j.apsusc.2012.02.080.
Fig. 2. Atomic % depth profile of (a) MIC poly-Ge and (b) SIC poly-Ge, as obtained by XPS.
removal of the Ni thin film. The SPC process involves a direct anneal without an Ni thin film after α -Ge deposition. Results and conclusions The Raman spectra of SIC poly-Ge, MIC poly-Ge, and SPC Ge are depicted in Fig. 1. There are clear Ge–Ge peaks in the cases of the SIC and MIC curves near 300 cm−1, demonstrating that poly-Ge thin films can be successfully fabricated by SIC and MIC. The SPC curve exhibits no peak at 300 cm−1, indicating that the SPC method is not capable of crystallizing amorphous germanium at 400 °C. The grain sizes of SIC poly-Ge and MIC poly-Ge were estimated by examining the full width at half maximum (FWHM) of the Ge–Ge peak near 300 cm−1. It is well known that the grain size is related to the FWHM: the smaller the FWHM, the larger the grain size of the poly-Ge film [10]. The FWHM of the SIC poly-Ge thin film was 8.22, and it was observed to be smaller than the FWHM of the MIC poly-Ge thin film that was prepared under similar conditions, which was 10.08. It indicates that the SIC method produced larger grains than those produced by the MIC method. To further analyze the properties of the Ge thin film, the crystallinity of the films was calculated using the following formula, which defines the crystalline fraction (Xc) using integral intensities as Ic/(Ic + γIa) [11]. Ic denotes the crystalline integral intensity near 300 cm−1, and Ia denotes the amorphous integral intensity near 270 cm−1. Further, the photo 2