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Realization of large-scale sub-10nm nanogratings using a repetitive wet-chemical oxidation and etching technique

Realization of large-scale sub-10nm nanogratings using a repetitive wet-chemical oxidation and... Despite many efforts to create nanogratings to exploit exceptionally enhanced nano ‑ sized effects, realizing sub ‑ 10 nm nanogratings on a large area still remains a great challenge. Herein, we demonstrate fabrication of an inch‑ scale sub‑ 10 nm nanogratings with simple and reliable features, by employing a repetitive wet‑ chemical oxidation and etching technique. We found that a few atomic layers of the silicon surface are naturally oxidized in a nitric acid (HNO ) solution, which enables the surface of the silicon to be etched with 1.0 nm resolution by selectively removing the oxide layers. By combining this high‑ precision etching technique with silicon nanogratings previously prepared by conventional methods, we successfully demonstrated a sub‑ 10 nm silicon nanogratings on an inch‑ scale wafer. Keywords: Nanograting, Sub‑ 10 nm, Large‑ scale, Self‑ limiting oxidation, Wet‑ chemical digital etching, Repetitive wet‑ chemical oxidation and etching To fabricate the nanograting substrate, researchers Background have suggested various methods based on conventional Nanogratings are a three-dimensional array of regularly photolithography, including KrF [8], immersion [9], spaced nanoscale line structures. They have received and spacer [10] lithography. Even though hundreds of a great deal of attention because their anisotropic form nanometer scale grating patterns have been successfully factor is desirable for high performance electrical [1] and fabricated, it is difficult to fabricate grating patterns with optical [2, 3] devices. The recent development of nano - dimensions of tens of nanometers because of resolution fabrication technologies has made it possible to fabricate limitations [11]. Improved technologies, such as direct nanowire- [4] and nanogap-arrays [5], utilizing the shape self-assembly [12, 13] and e-beam lithography [14], have of nanogratings, so that nanogratings have becoming been developed that can fabricate devices on the tens of increasingly useful for various applications. In particu- nanometer scale. However, they still have fabrication dif- lar, many researchers have found that the properties of ficulties including stability, and yield on large areas. a material can be dramatically improved when the pat- In a successful effort to produce tens of nanometer- tern size is reduced to tens of nanometers and below [6, scale nanogratings over a large area, we previously devel- 7]. There have been a considerable number of studies oped a pattern downscaling technology based on multiple focused on fabricating small nanogratings on large areas spacer lithography and pattern-recovery techniques [15]. to realize high performance devices. As a result, highly compact and continuous 100 nm pitch silicon nanogratings were successfully fabricated on 8-inch wafer, and the method allowed us to fabricate var- ious material-based nanowires easily with extremely high aspect ratio (4,000,000:1). However, this technique also *Correspondence: jbyoon@kaist.ac.kr † has a resolution limitation, which prevents it from being Min‑ Seung Jo and Kwang‑ Wook Choi contributed equally to this work School of Electrical Engineering, Korea Advanced Institute of Science used for superior nano-scale phenomena [6, 7]. Thus, it is and Technology (KAIST ), 291 Daehak‑ro, Yuseong‑gu, Daejeon 34141, still necessary to develop a method to fabricate a smaller Republic of Korea © The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Jo et al. Micro and Nano Syst Lett (2017) 5:19 Page 2 of 7 dimension nanogratings on a large area with stable and experiments because the temperature and concentration simple features. of the HNO solution stabilized at this azeotropic point. In this paper, we demonstrate a large-scale sub-10  nm Moreover, the highest throughput was achievable at this silicon nanogratings. To realize the sub-10  nm grat- point, since the oxidation process is accelerated at high ing, we developed a nitric acid (HNO ) based surface temperature. oxidation/etching technique, based on a conventional wet-chemical solution process. We determined that the Experiment moderate reactant used for silicon oxidation exists stably The overall process is schematically shown in Fig.  1. at the azeotropic point of HNO and the oxidation is self- The wet-chemical digital etching was carried out by limiting within a few nanometer range. As a result, ultra- repetitive HNO oxidation and oxide etching. First, the accurate silicon etching (1.0 nm level) can be achieved by HNO solution at azeotropic point should be prepared selectively removing the oxidized surface. for experiments. By heating any concentration of HNO By combining our multiple spacer lithography/pat- solution, it can reach the azeotropic point with change tern recovery technique and the developed HNO based of boiling point according to its concentration. When silicon etching techniques, we successfully fabricated the boiling point of HNO solution is kept constant at sub-10  nm silicon nanogratings on an inch-scale level. 120  °C, it means that HNO solution reached the azeo- The developed method is based on conventional photo - tropic point, so its concentration is kept constant at 68%. lithography and low temperature wet-chemical solutions While continuously heating the HNO solution on hot with nanoscale controllability, which is highly useful plate to keep the solution at the azeotropic point, nano- for achieving various nanostructures such as nanodots, structured substrate (100  nm pitch nanogratings) was nanosquares, and nanoshells, and their applications. immersed into the HNO solution for a sufficient time over 5  min until the oxide thickness is saturated. After Approach the oxidation process was finished, the shallow oxide The proposed method employed a nanoscale digital etch - was then removed using diluted hydrofluoric acid (HF, ing process, based on activating only the surface layer 0.1 wt%) solution. It is necessary to completely rinse the and then selectively removing the activated layer [16–18]. substrate after each process in order to prevent contami- Previously, oxidant solutions such as hydrogen peroxide nation. By alternately immersing the prepared nanograt- (H O ) and mixture of H O and acid solution have been ing substrate into these two solutions, the width of the 2 2 2 2 typically used as the oxidant solutions in the wet-chemi- nanogratings was progressively thinned as the number cal digital etching technique. However, they are not suit- of process cycles increased. The dimensions could be able for the pattern reduction of large-scale substrates, completely controlled, and were used to realize sub- because of their weak oxidizing power or non-uniform 10 nm structures. reactivity [19, 20]. To form an ultra-fine silicon oxide uniformly over a Results and discussion large area, we engineered the proper nitric acid (HNO ) The first objective was to identify the saturation time and solution to be used as the oxidant solution for the wet- thickness of the silicon in the HNO solution. This would chemical digital etching. HNO is a well-known oxidiz- reveal the resolution of the proposed method that could ing agent which generates nitric oxide (NO) and nitrogen be achieved in a single size reduction cycle. The oxide dioxide (NO ) with singlet oxygen as the reactive oxygen thickness was investigated by spectroscopic ellipsometer species. Silicon oxidation process using nitric acid follows (M2000D, Woolam) and high-resolution transmission self-limiting oxidation process in which the thickness electron microscopy (HR-TEM; TecnaiTM G2 F30 super- of the oxide is saturated and does scarcely change after twin, FEI), and the results are shown in Fig. 2. a certain period of time, and it is schematically shown As the oxidation time increased, the oxide thickness in bottom of Fig.  1. In the nitric acid oxidation process, continued to increase until it reached the saturation diffusion of this reactive oxygen species promotes oxida - point at about 5 min. After 5 min of oxidation, the oxide tion of the surface of the silicon, until the diffusion of the thickness saturated at around 1.8  nm and we confirmed oxidizing agent equilibrates with the stress induced by that further oxidation up to 30  min did not increase the volume expansion during the oxidation process [21–23]. thickness. The saturated oxide thickness after 5  min of This saturates the oxide thickness at a fixed and predict - oxidation was also confirmed using HR-TEM imagery, able value. To guarantee accuracy and ensure the reli- and the results agreed with the value obtained from spec- able growth of the saturated oxide, the azeotropic point troscopic ellipsometry. Based on this identification, we (120 °C, 68 wt%) of the HNO solution was utilized [24]. set the oxidation time to over 5  min for each repeated Process conditions could be maintained throughout the oxidation cycle. Jo et al. Micro and Nano Syst Lett (2017) 5:19 Page 3 of 7 Fig. 1 Schematics of the fabrication process. a Ultra‑thin nanogratings were produced from initial nanogratings by repetitive oxidation and etching cycles. One cycle consists of an oxidation process in nitric acid and an etching process in diluted hydrofluoric acid. b Growth of silicon oxide with respect to time in the oxidation process; silicon oxide grows until the saturation point, and then remains at a constant saturated thickness ( Tox, sat) after this saturation time Fig. 2 Measurement of silicon oxide thickness. a Silicon oxide thickness with respect to oxidation time, measured by ellipsometer, with an extended line at 1.8 nm. b A high resolution TEM image (×790,000) of the Si/SiO interface 2 Jo et al. Micro and Nano Syst Lett (2017) 5:19 Page 4 of 7 The accurate resolution of the proposed method was surfaces, no noticeable change could be observed. This established by measuring the etching depth of the silicon means the silicon surface was uniformly oxidized and based on the number of repeated oxidation and etch- etched with each repeated cycle. ing cycles. Using a flat silicon wafer with a patterned Cr The proposed size reduction method was then applied masking layer, the surface of the silicon was selectively to fabricate nanogratings. The prepared substrate had a oxidized and etched for several cycles. The resulting 100 nm pitch nanograting pattern with a 40 nm line and depth profile was measured using atomic force micros - a 60 nm space, as a scanning electron microscope (SEM; copy (AFM; XE-100, PSIA), and the detailed experiment Sirion, FEI) image shown in Fig.  4a. After the substrate method is shown in Fig. 3. was immersed in the HNO solution for 5 min to form a AFM measurement was carried out after repeating saturated oxide layer, TEM observation was carried out. the size reduction cycles 5, 10, 15, 20 times respectively. The cross-sectional TEM images shown in Fig.  4b indi- As expected, the depth increased proportionally with cate the entire surface of the nanogratings was uniformly the number of repeated cycles, and the extracted etch- oxidized, which can be attributed to the self-limiting oxi- ing depth per cycle from linear fitting was 1.0  nm. Note dation process of the wet-chemical oxidizing agent. We that the smaller etching depth of silicon compared with can also infer from these results that only the width of the the produced oxide thickness can be explained with the nanogratings can be thinned as the size reduction cycles conventional model of silicon volume expansion during progress, while other dimensions, such as the height and the oxidation process. As can be seen in Fig.  3a, about pitch of the nanogratings are maintained. half of the oxide is formed under the silicon surface. This Finally, the decrease in the line width of the nanogratings phenomenon is well-known and originated from the fact based on the increasing number of cycles is illustrated in that oxide molecular density is about half of that of sili- Fig. 5. The width distribution was investigated statistically con. This is why the silicon etching depth is about half by measuring the length of 50 peaks of the nanogratings. of the oxide thickness [25]. Furthermore, when surface Every five additional cycles reduced the line width of the roughness was compared between the initial and etched nanogratings by about 10  nm. This agrees with the AFM Fig. 3 AFM data confirming etch‑ depth and roughness after the oxidation and etching cycles. a Schematic of the process in a flat silicon substrate to be analyzed by AFM. b Etch depth means the difference in height between the surface of the initial flat substrate and the surface of the etched silicon, according to the number of oxidation and etching cycles (Additional file 1). c AFM topographical images, before and after 20 cycles. Rough‑ ness is the root mean square of the surface Jo et al. Micro and Nano Syst Lett (2017) 5:19 Page 5 of 7 Fig. 4 Wet‑ chemical oxidation of nanogratings. a A schematic illustration of the nanogratings characterizing the 100 nm pitch with sub‑50 nm line patterns; and an SEM image of the side view of a piece of the nanogratings showing the 40 nm features. b TEM images showing a cross section of one pattern of the nanogratings after oxidation for 5 min (left) and the corresponding magnification of peaks and valley area (right) Fig. 5 Various sizes of nanogratings produced by the wet‑ chemical etching technique. SEM image of top view and cross section of the nanogratings in the initial state, after 5, 10 and 15 cycles; and size distribution as measured by the corresponding SEM images above (Additional file 2), with the standard deviation (σ) and the coefficient of variation (CV ) (the standard deviation/the mean) Jo et al. Micro and Nano Syst Lett (2017) 5:19 Page 6 of 7 Fig. 6 Demonstration of the inch‑scale sub ‑10 nm nanogratings. a Optical image of the large substrate, 2.0 cm × 2.5 cm after 15 cycles, and SEM images of a top view of the corresponding areas numbered in the optical image. b Difference in size deviation between before and after 15 cycles for each numbered position (Additional file 2) measurement results, that each cycle reduced both side- etching technique can remove silicon at a resolution of walls of the nanogratings by 1.0 nm. For each cycle, about 1.0  nm per cycle, because the HNO oxidizes only a few 2 nm of width was reduced from the original pattern. With atomic layers of silicon surface per cycle. Saturation of this high level of resolution, the dimensions of the nano- the silicon oxide thickness means that this technique can structure could be precisely controlled and thinned to the be used to uniformly etch the surface of a large area. A targeted sub-10  nm size, by determining the number of previously prepared sub-40  nm nanogratings on inch- size reduction cycles. In addition, the standard deviation of scale wafer was reduced to a precisely controlled sub- the pattern size decreases as increasing the number of oxi- 10  nm using this technique. Furthermore, the nanoscale dation and etching cycles, and the coefficient of variation, size reduction technique developed in this work can be on the other hand, increases as the number of processes. applied to any kind of silicon nanostructures, in addition The reason of this tendency requires further investigation. to nanogratings. The sub-10  nm structures produced by However, since the standard deviation stays within 2  nm this method have great potential for a variety of applica- and the coefficient of variation (the standard deviation/the tions in high performance electronics, optoelectronics and mean) is about 10% at 15 cycles of etching, it means that sensing devices. sub-10 nm patterns could be manufactured stably through the proposed method. Additional files To verify the large-scale applicability of the proposed method, we conducted the same experiments on a 2  cm Additional file 1. Z ‑ drive profiles measured by AFM; the datasets state by 2.5 cm area and observed the position dependent man- the measured values of graph in Fig. 3b. ufacturing result. Optical and SEM images were obtained Additional file 2. Pattern size distribution of nanogratings measured from SEM images; the datasets state statistical values of graphs in Fig. 5 after applying the size reduction method for 15 cycles, and Fig. 6b. as illustrated in Fig.  6. The statistical investigation of 20 peaks of the nanogratings for each divided position con- firmed that all of the surfaces were uniformly reduced, Authors’ contributions MSJ and KWC carried out design, fabrication, measurement and analysis of regardless of their position on the wafer. This result is the results, and drafted the manuscript. SMH performed analysis of results and again attributed to the use of the self-limiting oxidation of participated in editing the manuscript. All authors read and approved the final the wet-chemical oxidant in our proposed method. manuscript. Acknowledgements Conclusion We also thanks to members of our laboratory (3D micro‑nano structures lab.) We realized a sub-10  nm nanogratings on a large-scale for sincere comments on this research. wafer by employing a wet-chemical digital etching tech- Competing interests nique. The introduced HNO based wet-chemical digital The authors declare that they have no competing interests. Jo et al. Micro and Nano Syst Lett (2017) 5:19 Page 7 of 7 Availability of data and materials 11. Ito T, Okazaki S (2000) Pushing the limits of lithography. Nature The datasets supporting the conclusions of this article are included within the 406:1027–1031 article and its additional files. 12. Cheng JY, Rettner CT, Sanders DP, Kim HC, Hinsberg WD (2008) Dense self‑assembly on sparse chemical patterns: rectifying and multiplying Funding lithographic patterns using block copolymers. Adv Mater 20:3155–3158 This work was supported by a National Research Foundation of Korea (NRF) 13. Park S‑M, Liang X, Harteneck BD, Pick TE, Hiroshiba N, Wu Y, Helms BA, Grant funded by the Korean government (MEST ) (No. 2011‑0028781). Olynick DL (2011) Sub‑10 nm nanofabrication via nanoimprint directed self‑assembly of block copolymers. ACS Nano 5:8523–8531 Received: 7 December 2016 Accepted: 19 March 2017 14. Joo J, Chow BY, Jacobson JM (2006) Nanoscale patterning on insulat‑ ing substrates by critical energy electron beam lithography. Nano Lett 6:2021–2025 15. Yeon J et al (2013) High throughput ultralong (20 cm) nanowire fabrica‑ tion using a wafer‑scale nanograting template. Nano Lett 13:3978–3984 16. Ishii M, Meguro T, Gamo K, Sugano T, Aoyagi Y (1993) Digital etching References using Krf excimer laser: approach to atomic‑ order‑ controlled etching by 1. Choi YK, Lindert N, Xuan P, Tang S, Ha D, Anderson E, King TJ, Bokor J, Hu C photo induced reaction. Jpn J Appl Phys 32:6178 (2001) Sub‑20 nm Cmos Finfet technologies. IEEE, New York 17. Meguro T, Ishii M, Kodama K, Yamamoto Y, Gamo K, Aoyagi Y (1993) 2. Weber T, Kroker S, Käsebier T, Kley EB, Tünnermann A (2014) Silicon wire Surface processes in digital etching of gaas. Thin Solid Films 225:136–139 grid polarizer for ultraviolet applications. Appl Opt 53:8140–8144 18. Sakaue H, Iseda S, Asami K, Yamamoto J, Hirose M, Horiike Y (1990) 3. You J, Li X, Xie FX, Sha WE, Kwong JH, Li G, Choy WC, Yang Y (2012) Sur‑ Atomic layer controlled digital etching of silicon. Jpn J Appl Phys 29:2648 face plasmon and scattering‑ enhanced low‑bandgap polymer solar cell 19. DeSalvo GC, Bozada CA, Ebel JL, Look DC, Barrette JP, Cerny CL, Dettmer by a metal grating back electrode. Adv Energy Mater 2:1203–1207 RW, Gillespie JK, Havasy CK, Jenkins TJ (1996) Wet chemical digital etching 4. Jeong JW et al (2014) High‑resolution nanotransfer printing applicable to of gaas at room temperature. J Electrochem Soc 143:3652–3656 diverse surfaces via interface‑targeted adhesion switching. Nat Commun 20. Hennessy K, Badolato A, Tamboli A, Petroff PM, Hu E, Atatüre M, Dreiser 5:5387 J, Imamoğlu A (2005) Tuning photonic crystal nanocavity modes by wet 5. Siegfried T, Ekinci Y, Solak HH, Martin OJF, Sigg H (2011) Fabrication of chemical digital etching. Appl Phys Lett 87:021108 sub‑10 nm gap arrays over large areas for plasmonic sensors. Appl Phys 21. Deal BE, Grove AS (1965) General relationship for the thermal oxidation of Lett 99:263302 silicon. J Appl Phys 36:3770 6. Hodes G (2007) When small is different: some recent advances in con‑ 22. Fazzini P‑F, Bonafos C, Claverie A, Hubert A, Ernst T, Respaud M (2011) cepts and applications of nanoscale phenomena. Adv Mater 19:639–655 Modeling stress retarded self‑limiting oxidation of suspended silicon 7. Ieong M, Doris B, Kedzierski J, Rim K, Yang M (2004) Silicon device scaling nanowires for the development of silicon nanowire‑based nanodevices. J to the sub‑10‑nm regime. Science 306:2057–2060 Appl Phys 110:033524 8. Fritze M, Tyrrell B, Astolfi DK, Yost D, Davis P, Wheeler B, Mallen RD, 23. Liu HI, Biegelsen DK, Ponce FA, Johnson NM, Pease RFW (1994) Self‑ Jarmolowicz J, Cann S, GLiu HY (2001). 100‑nm node lithography with limiting oxidation for fabricating sub‑5 nm silicon nanowires. Appl Phys Krf?. In 26th Annual international symposium on microlithography, inter‑ Lett 64:1383 national society for optics and photonics, pp 191–204 24. Kobayashi Asuha H, Maida O, Takahashi M, Iwasa H (2003) Nitric acid 9. Switkes M, Rothschild M (2001) Immersion lithography at 157 nm. J Vac oxidation of si to form ultrathin silicon dioxide layers with a low leakage Sci Technol B 19:2353–2356 current density. J Appl Phys 94:7328 10. Jung WY, Kim CD, Eom JD, Cho SY, Jeon SM, Kim JH, Moon JI, Lee BS, 25. Hu SM (1991) Stress‑related problems in silicon technology. J Appl Phys Park SK (2006). Patterning with spacer for expanding the resolution limit 70:R53–R80 of current lithography tool. In SPIE 31st international symposium on advanced lithography, international society for optics and photonics, pp 61561J–61561J‑9 http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Micro and Nano Systems Letters Springer Journals

Realization of large-scale sub-10nm nanogratings using a repetitive wet-chemical oxidation and etching technique

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Springer Journals
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Copyright © 2017 by The Author(s)
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Engineering; Circuits and Systems; Electrical Engineering; Mechanical Engineering; Nanotechnology; Applied and Technical Physics
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Abstract

Despite many efforts to create nanogratings to exploit exceptionally enhanced nano ‑ sized effects, realizing sub ‑ 10 nm nanogratings on a large area still remains a great challenge. Herein, we demonstrate fabrication of an inch‑ scale sub‑ 10 nm nanogratings with simple and reliable features, by employing a repetitive wet‑ chemical oxidation and etching technique. We found that a few atomic layers of the silicon surface are naturally oxidized in a nitric acid (HNO ) solution, which enables the surface of the silicon to be etched with 1.0 nm resolution by selectively removing the oxide layers. By combining this high‑ precision etching technique with silicon nanogratings previously prepared by conventional methods, we successfully demonstrated a sub‑ 10 nm silicon nanogratings on an inch‑ scale wafer. Keywords: Nanograting, Sub‑ 10 nm, Large‑ scale, Self‑ limiting oxidation, Wet‑ chemical digital etching, Repetitive wet‑ chemical oxidation and etching To fabricate the nanograting substrate, researchers Background have suggested various methods based on conventional Nanogratings are a three-dimensional array of regularly photolithography, including KrF [8], immersion [9], spaced nanoscale line structures. They have received and spacer [10] lithography. Even though hundreds of a great deal of attention because their anisotropic form nanometer scale grating patterns have been successfully factor is desirable for high performance electrical [1] and fabricated, it is difficult to fabricate grating patterns with optical [2, 3] devices. The recent development of nano - dimensions of tens of nanometers because of resolution fabrication technologies has made it possible to fabricate limitations [11]. Improved technologies, such as direct nanowire- [4] and nanogap-arrays [5], utilizing the shape self-assembly [12, 13] and e-beam lithography [14], have of nanogratings, so that nanogratings have becoming been developed that can fabricate devices on the tens of increasingly useful for various applications. In particu- nanometer scale. However, they still have fabrication dif- lar, many researchers have found that the properties of ficulties including stability, and yield on large areas. a material can be dramatically improved when the pat- In a successful effort to produce tens of nanometer- tern size is reduced to tens of nanometers and below [6, scale nanogratings over a large area, we previously devel- 7]. There have been a considerable number of studies oped a pattern downscaling technology based on multiple focused on fabricating small nanogratings on large areas spacer lithography and pattern-recovery techniques [15]. to realize high performance devices. As a result, highly compact and continuous 100 nm pitch silicon nanogratings were successfully fabricated on 8-inch wafer, and the method allowed us to fabricate var- ious material-based nanowires easily with extremely high aspect ratio (4,000,000:1). However, this technique also *Correspondence: jbyoon@kaist.ac.kr † has a resolution limitation, which prevents it from being Min‑ Seung Jo and Kwang‑ Wook Choi contributed equally to this work School of Electrical Engineering, Korea Advanced Institute of Science used for superior nano-scale phenomena [6, 7]. Thus, it is and Technology (KAIST ), 291 Daehak‑ro, Yuseong‑gu, Daejeon 34141, still necessary to develop a method to fabricate a smaller Republic of Korea © The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Jo et al. Micro and Nano Syst Lett (2017) 5:19 Page 2 of 7 dimension nanogratings on a large area with stable and experiments because the temperature and concentration simple features. of the HNO solution stabilized at this azeotropic point. In this paper, we demonstrate a large-scale sub-10  nm Moreover, the highest throughput was achievable at this silicon nanogratings. To realize the sub-10  nm grat- point, since the oxidation process is accelerated at high ing, we developed a nitric acid (HNO ) based surface temperature. oxidation/etching technique, based on a conventional wet-chemical solution process. We determined that the Experiment moderate reactant used for silicon oxidation exists stably The overall process is schematically shown in Fig.  1. at the azeotropic point of HNO and the oxidation is self- The wet-chemical digital etching was carried out by limiting within a few nanometer range. As a result, ultra- repetitive HNO oxidation and oxide etching. First, the accurate silicon etching (1.0 nm level) can be achieved by HNO solution at azeotropic point should be prepared selectively removing the oxidized surface. for experiments. By heating any concentration of HNO By combining our multiple spacer lithography/pat- solution, it can reach the azeotropic point with change tern recovery technique and the developed HNO based of boiling point according to its concentration. When silicon etching techniques, we successfully fabricated the boiling point of HNO solution is kept constant at sub-10  nm silicon nanogratings on an inch-scale level. 120  °C, it means that HNO solution reached the azeo- The developed method is based on conventional photo - tropic point, so its concentration is kept constant at 68%. lithography and low temperature wet-chemical solutions While continuously heating the HNO solution on hot with nanoscale controllability, which is highly useful plate to keep the solution at the azeotropic point, nano- for achieving various nanostructures such as nanodots, structured substrate (100  nm pitch nanogratings) was nanosquares, and nanoshells, and their applications. immersed into the HNO solution for a sufficient time over 5  min until the oxide thickness is saturated. After Approach the oxidation process was finished, the shallow oxide The proposed method employed a nanoscale digital etch - was then removed using diluted hydrofluoric acid (HF, ing process, based on activating only the surface layer 0.1 wt%) solution. It is necessary to completely rinse the and then selectively removing the activated layer [16–18]. substrate after each process in order to prevent contami- Previously, oxidant solutions such as hydrogen peroxide nation. By alternately immersing the prepared nanograt- (H O ) and mixture of H O and acid solution have been ing substrate into these two solutions, the width of the 2 2 2 2 typically used as the oxidant solutions in the wet-chemi- nanogratings was progressively thinned as the number cal digital etching technique. However, they are not suit- of process cycles increased. The dimensions could be able for the pattern reduction of large-scale substrates, completely controlled, and were used to realize sub- because of their weak oxidizing power or non-uniform 10 nm structures. reactivity [19, 20]. To form an ultra-fine silicon oxide uniformly over a Results and discussion large area, we engineered the proper nitric acid (HNO ) The first objective was to identify the saturation time and solution to be used as the oxidant solution for the wet- thickness of the silicon in the HNO solution. This would chemical digital etching. HNO is a well-known oxidiz- reveal the resolution of the proposed method that could ing agent which generates nitric oxide (NO) and nitrogen be achieved in a single size reduction cycle. The oxide dioxide (NO ) with singlet oxygen as the reactive oxygen thickness was investigated by spectroscopic ellipsometer species. Silicon oxidation process using nitric acid follows (M2000D, Woolam) and high-resolution transmission self-limiting oxidation process in which the thickness electron microscopy (HR-TEM; TecnaiTM G2 F30 super- of the oxide is saturated and does scarcely change after twin, FEI), and the results are shown in Fig. 2. a certain period of time, and it is schematically shown As the oxidation time increased, the oxide thickness in bottom of Fig.  1. In the nitric acid oxidation process, continued to increase until it reached the saturation diffusion of this reactive oxygen species promotes oxida - point at about 5 min. After 5 min of oxidation, the oxide tion of the surface of the silicon, until the diffusion of the thickness saturated at around 1.8  nm and we confirmed oxidizing agent equilibrates with the stress induced by that further oxidation up to 30  min did not increase the volume expansion during the oxidation process [21–23]. thickness. The saturated oxide thickness after 5  min of This saturates the oxide thickness at a fixed and predict - oxidation was also confirmed using HR-TEM imagery, able value. To guarantee accuracy and ensure the reli- and the results agreed with the value obtained from spec- able growth of the saturated oxide, the azeotropic point troscopic ellipsometry. Based on this identification, we (120 °C, 68 wt%) of the HNO solution was utilized [24]. set the oxidation time to over 5  min for each repeated Process conditions could be maintained throughout the oxidation cycle. Jo et al. Micro and Nano Syst Lett (2017) 5:19 Page 3 of 7 Fig. 1 Schematics of the fabrication process. a Ultra‑thin nanogratings were produced from initial nanogratings by repetitive oxidation and etching cycles. One cycle consists of an oxidation process in nitric acid and an etching process in diluted hydrofluoric acid. b Growth of silicon oxide with respect to time in the oxidation process; silicon oxide grows until the saturation point, and then remains at a constant saturated thickness ( Tox, sat) after this saturation time Fig. 2 Measurement of silicon oxide thickness. a Silicon oxide thickness with respect to oxidation time, measured by ellipsometer, with an extended line at 1.8 nm. b A high resolution TEM image (×790,000) of the Si/SiO interface 2 Jo et al. Micro and Nano Syst Lett (2017) 5:19 Page 4 of 7 The accurate resolution of the proposed method was surfaces, no noticeable change could be observed. This established by measuring the etching depth of the silicon means the silicon surface was uniformly oxidized and based on the number of repeated oxidation and etch- etched with each repeated cycle. ing cycles. Using a flat silicon wafer with a patterned Cr The proposed size reduction method was then applied masking layer, the surface of the silicon was selectively to fabricate nanogratings. The prepared substrate had a oxidized and etched for several cycles. The resulting 100 nm pitch nanograting pattern with a 40 nm line and depth profile was measured using atomic force micros - a 60 nm space, as a scanning electron microscope (SEM; copy (AFM; XE-100, PSIA), and the detailed experiment Sirion, FEI) image shown in Fig.  4a. After the substrate method is shown in Fig. 3. was immersed in the HNO solution for 5 min to form a AFM measurement was carried out after repeating saturated oxide layer, TEM observation was carried out. the size reduction cycles 5, 10, 15, 20 times respectively. The cross-sectional TEM images shown in Fig.  4b indi- As expected, the depth increased proportionally with cate the entire surface of the nanogratings was uniformly the number of repeated cycles, and the extracted etch- oxidized, which can be attributed to the self-limiting oxi- ing depth per cycle from linear fitting was 1.0  nm. Note dation process of the wet-chemical oxidizing agent. We that the smaller etching depth of silicon compared with can also infer from these results that only the width of the the produced oxide thickness can be explained with the nanogratings can be thinned as the size reduction cycles conventional model of silicon volume expansion during progress, while other dimensions, such as the height and the oxidation process. As can be seen in Fig.  3a, about pitch of the nanogratings are maintained. half of the oxide is formed under the silicon surface. This Finally, the decrease in the line width of the nanogratings phenomenon is well-known and originated from the fact based on the increasing number of cycles is illustrated in that oxide molecular density is about half of that of sili- Fig. 5. The width distribution was investigated statistically con. This is why the silicon etching depth is about half by measuring the length of 50 peaks of the nanogratings. of the oxide thickness [25]. Furthermore, when surface Every five additional cycles reduced the line width of the roughness was compared between the initial and etched nanogratings by about 10  nm. This agrees with the AFM Fig. 3 AFM data confirming etch‑ depth and roughness after the oxidation and etching cycles. a Schematic of the process in a flat silicon substrate to be analyzed by AFM. b Etch depth means the difference in height between the surface of the initial flat substrate and the surface of the etched silicon, according to the number of oxidation and etching cycles (Additional file 1). c AFM topographical images, before and after 20 cycles. Rough‑ ness is the root mean square of the surface Jo et al. Micro and Nano Syst Lett (2017) 5:19 Page 5 of 7 Fig. 4 Wet‑ chemical oxidation of nanogratings. a A schematic illustration of the nanogratings characterizing the 100 nm pitch with sub‑50 nm line patterns; and an SEM image of the side view of a piece of the nanogratings showing the 40 nm features. b TEM images showing a cross section of one pattern of the nanogratings after oxidation for 5 min (left) and the corresponding magnification of peaks and valley area (right) Fig. 5 Various sizes of nanogratings produced by the wet‑ chemical etching technique. SEM image of top view and cross section of the nanogratings in the initial state, after 5, 10 and 15 cycles; and size distribution as measured by the corresponding SEM images above (Additional file 2), with the standard deviation (σ) and the coefficient of variation (CV ) (the standard deviation/the mean) Jo et al. Micro and Nano Syst Lett (2017) 5:19 Page 6 of 7 Fig. 6 Demonstration of the inch‑scale sub ‑10 nm nanogratings. a Optical image of the large substrate, 2.0 cm × 2.5 cm after 15 cycles, and SEM images of a top view of the corresponding areas numbered in the optical image. b Difference in size deviation between before and after 15 cycles for each numbered position (Additional file 2) measurement results, that each cycle reduced both side- etching technique can remove silicon at a resolution of walls of the nanogratings by 1.0 nm. For each cycle, about 1.0  nm per cycle, because the HNO oxidizes only a few 2 nm of width was reduced from the original pattern. With atomic layers of silicon surface per cycle. Saturation of this high level of resolution, the dimensions of the nano- the silicon oxide thickness means that this technique can structure could be precisely controlled and thinned to the be used to uniformly etch the surface of a large area. A targeted sub-10  nm size, by determining the number of previously prepared sub-40  nm nanogratings on inch- size reduction cycles. In addition, the standard deviation of scale wafer was reduced to a precisely controlled sub- the pattern size decreases as increasing the number of oxi- 10  nm using this technique. Furthermore, the nanoscale dation and etching cycles, and the coefficient of variation, size reduction technique developed in this work can be on the other hand, increases as the number of processes. applied to any kind of silicon nanostructures, in addition The reason of this tendency requires further investigation. to nanogratings. The sub-10  nm structures produced by However, since the standard deviation stays within 2  nm this method have great potential for a variety of applica- and the coefficient of variation (the standard deviation/the tions in high performance electronics, optoelectronics and mean) is about 10% at 15 cycles of etching, it means that sensing devices. sub-10 nm patterns could be manufactured stably through the proposed method. Additional files To verify the large-scale applicability of the proposed method, we conducted the same experiments on a 2  cm Additional file 1. Z ‑ drive profiles measured by AFM; the datasets state by 2.5 cm area and observed the position dependent man- the measured values of graph in Fig. 3b. ufacturing result. Optical and SEM images were obtained Additional file 2. Pattern size distribution of nanogratings measured from SEM images; the datasets state statistical values of graphs in Fig. 5 after applying the size reduction method for 15 cycles, and Fig. 6b. as illustrated in Fig.  6. The statistical investigation of 20 peaks of the nanogratings for each divided position con- firmed that all of the surfaces were uniformly reduced, Authors’ contributions MSJ and KWC carried out design, fabrication, measurement and analysis of regardless of their position on the wafer. This result is the results, and drafted the manuscript. SMH performed analysis of results and again attributed to the use of the self-limiting oxidation of participated in editing the manuscript. All authors read and approved the final the wet-chemical oxidant in our proposed method. manuscript. Acknowledgements Conclusion We also thanks to members of our laboratory (3D micro‑nano structures lab.) 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Journal

Micro and Nano Systems Letters – Springer Journals

Published: Mar 24, 2017

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