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Graphitized silicon carbide microbeams: wafer-level, self-aligned graphene on silicon wafers
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 325301 (http://iopscience.iop.org/0957-4484/25/32/325301) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 130.130.211.200 This content was downloaded on 16/07/2017 at 21:13 Please note that terms and conditions apply.
You may also be interested in: Solid source growth of graphene with Ni–Cu catalysts: towards high quality in situ graphene on silicon Neeraj Mishra, John J Boeckl, Anton Tadich et al. A thin film approach for SiC-derived graphene as an on-chip electrode for supercapacitors Mohsin Ahmed, Mohamad Khawaja, Marco Notarianni et al. Combining graphene with silicon carbide: synthesis and properties – a review Ivan Shtepliuk, Volodymyr Khranovskyy and Rositsa Yakimova High-quality, single-layered epitaxial graphene fabricated on 6H-SiC (0001) by flash annealing in Pb atmosphere and mechanism T W Hu, X T Liu, F Ma et al. Review on mechanism of directly fabricating wafer-scale graphene on dielectric substrates by chemical vapor deposition Jing Ning, Dong Wang, Yang Chai et al. Tension assisted metal transfer of graphene for Schottky diodes onto wafer scale substrates Jooho Lee, Su Chan Lee, Yongsung Kim et al. Fabrication of a Schottky junction diode with direct growth graphene on silicon by a solid phase reaction Golap Kalita, Ryo Hirano, Muhammed E Ayhan et al. Low temperature metal free growth of graphene on insulating substrates by plasma assisted chemical vapor deposition R Muñoz, C Munuera, J I Martínez et al.
Nanotechnology Nanotechnology 25 (2014) 325301 (7pp)
doi:10.1088/0957-4484/25/32/325301
Graphitized silicon carbide microbeams: wafer-level, self-aligned graphene on silicon wafers Benjamin V Cunning1, Mohsin Ahmed1, Neeraj Mishra1, Atieh Ranjbar Kermany1, Barry Wood2 and Francesca Iacopi1 1
Queensland Micro- and Nanotechnology Centre, Griffith University, Nathan 4111, Queensland, Australia Centre for Microscopy and Microanalysis, The University of Queensland, St. Lucia 4072, Queensland, Australia
2
E-mail: f.iacopi@griffith.edu.au Received 10 March 2014, revised 10 April 2014 Accepted for publication 22 May 2014 Published 23 July 2014 Abstract
Currently proven methods that are used to obtain devices with high-quality graphene on silicon wafers involve the transfer of graphene flakes from a growth substrate, resulting in fundamental limitations for large-scale device fabrication. Moreover, the complex three-dimensional structures of interest for microelectromechanical and nanoelectromechanical systems are hardly compatible with such transfer processes. Here, we introduce a methodology for obtaining thousands of microbeams, made of graphitized silicon carbide on silicon, through a site-selective and wafer-scale approach. A Ni-Cu alloy catalyst mediates a self-aligned graphitization on prepatterned SiC microstructures at a temperature that is compatible with silicon technologies. The graphene nanocoating leads to a dramatically enhanced electrical conductivity, which elevates this approach to an ideal method for the replacement of conductive metal films in silicon carbide-based MEMS and NEMS devices. S Online supplementary data available from stacks.iop.org/NANO/25/325301/mmedia Keywords: graphene, silicon carbide, self-aligned fabrication, MEMS (Some figures may appear in colour only in the online journal) 1. Introduction
encountered in microelectromechanical and nanoelectromechanical systems (MEMS & NEMS) unrealistic. Epitaxial silicon carbide (SiC) films on silicon (Si) are an ideal platform for the integration of graphene-coated structures for MEMS or NEMS applications, since crystalline SiC has outstanding mechanical properties [6, 7] and is a promising template for graphene growth through solid carbon source processes [8–13]. As patterning of the SiC on Si can be carried out through well-established photolithography and etching processes [14], the synergy of these properties allows for well-defined graphitized structures when using a selfaligned approach with critical dimensions that potentially reach down to a few nanometers. Currently, the most realistic route toward transfer-free, device-quality graphene on prepatterned solid carbon sources is based on silicon sublimation from SiC bulk wafers at very
Graphene is heralded as one of the most promising nanomaterials for a variety of applications, due to the extraordinary physical, optical, electronic and mechanical properties of its pristine form [1–3]. However, the massive impact that is expected from the utilization of such properties in actual microdevices or nanodevices at an extensive scale is still far from being a reality, due to hurdles in their replication in well-defined structures and to a substantial variability of device properties when using transferred graphene [4, 5]. Furthermore, the reliance on the manipulation of exfoliated or grown graphene flakes to realize devices does not allow for precisely defined positioning and dimensions, thereby making graphene integration with the complex structures commonly 0957-4484/14/325301+07$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
Nanotechnology 25 (2014) 325301
B V Cunning
Figure 1. Sequential steps for the wafer-level fabrication of graphitized silicon carbide microbeams on a silicon substrate. Once the SiC is
patterned, the few-layer graphene is grown selectively on the SiC structures via metal-mediated graphitization. The reacted metal layer is subsequently removed, and the structures are released from the substrate to form suspended beams.
Ni/SiC interface [10, 11], while the majority of people observed the graphene on the Ni surface [12, 13, 23, 24]. The latter case still clearly requires manual transfer of the graphene onto a semiconductor or an insulating surface to obtain a functional device, which hinders its utility for large-scale device fabrication. Herein, we build on the initial efforts of nickel mediated catalytic graphitization from SiC and show substantial advances through the use of epitaxial SiC on Si. By using a Ni-Cu alloy as opposed to a pure nickel catalyst, we achieve graphene growth directly on the SiC surface, thus eliminating the need for a manual graphene transfer, while concurrently improving the graphene crystallite size. We subsequently demonstrate a simple wafer-scale fabrication procedure, which validates the site-selective nature of the graphene growth and simultaneously realizes thousands of suspended SiC microstructures with a self-aligned nanocoating of fewlayer graphene. Electrical measurements of the obtained
high temperatures (1300–1600 °C) [15–18]. The prohibitive cost of the SiC wafers, as well as their patterning and micromachining, is a major limitation [1]. Attempts to replicate the success of this methodology through sublimation from a more affordable epitaxial SiC on silicon wafers have invariably shown substantially inferior graphene quality [19–22]. An alternative method for obtaining graphene from SiC involves a nickel-mediated catalytic graphitization at the SiC surface [9–12]. This approach takes place at temperatures ranging from 750 °C to 1200 °C, which is much lower than the sublimation process. However, the approach is not without its disadvantages. Earlier attempts at nickel-mediated graphitization from amorphous or crystalline SiC films on silicon have shown promise; however, the resulting graphene quality varies considerably. Additionally, there are conflicting reports regarding the location where the graphene was found. A few reports indicate that the graphene was obtained at the 2
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Figure 2. (a) The optical image after graphene growth on the SiC with the conventional single metal approach (Ni) shows large-scale non-
uniformity. The Raman spectrum from one of the bright regions, indicated by the circle, indicates high graphene defectivity (high D/G band ratio). (b) Optical image and typical Raman spectrum after graphene growth on the SiC using a Ni-Cu alloy in the form of a bilayer catalyst. Substantial improvements in both defectivity and uniformity are shown. The 2D/G Raman band ratio indicates bilayer graphene, whilst the D/G ratio of 0.5 reveals significantly lower defectivity when compared to figure 2(a).
thickness of ≈7 nm Ni and ≈15, 30 or 45 nm Cu, which was measured using a DecTak stylus profiler. For graphitization, the wafer was transferred into a Carbolite tube furnace and evacuated to a pressure below 1 × 10−3 mbar. The wafer was heated and maintained at 1050 °C for one hour and left to cool to room temperature under vacuum. The metal, metal silicides and carbon formed on the metal surface were all removed via sonication in a Freckle’s etch solution (70:10:5:5:10–85% H3PO4: Glacial acetic acid: 70% HNO3: 50% HBF4:H2O).
graphene point to our graphitization method as an ideal approach for the replacement of conducting layers in MEMS devices, in which metal films may degrade mechanical behavior, such as in quality factors for resonators [25].
2. Experimental 2.1. Graphitization
Epitaxial monocrystalline 3C-SiC layers, 250 nm thick, were grown onto two-inch Si(111) substrates in a custom-made, horizontal, hot wall, low-pressure chemical vapour furnace. The SiC films were grown at 1000 °C via an alternate supply of SiH4 and C3H6 and resulted in unintentionally n-type doped films, with a doping level around 1016–1017 at cm−3, as earlier described [26]. Thin layers of first nickel (99.95%), followed by copper (99.95%), were deposited using a DC Ar+ ion sputterer with a deposition current of 100 mA at a base pressure of 5 × 10−2 mbar. Nickel was sputtered for 10 s, and copper was sputtered at either 10, 20 or 30 s, yielding a
2.2. SiC patterning
The fabrication of graphitized microbeams proceeded through a self-aligned graphitization onto prepatterned SiC structures following the flow illustrated in figure 1. Conventional proximity photolithography with an AZ 6612 photoresist was used to define the regions of SiC to be etched in order to generate the structures. HCl plasma was used to selectively etch the SiC, and the remaining photoresist was removed with an O2 plasma etch. Graphitization then 3
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proceeded as per 2.1. The release of the SiC microstructures from the bulk Si substrate was achieved through a dry etch with XeF2, which etched anisotropically underneath the patterned SiC [7].
3. Results and discussion 3.1. Graphene synthesis and characterization
Raman spectra and corresponding optical images for films prepared with both nickel only and with the nickel/copper bilayer (1.66 Cu:Ni thickness ratio) are compared in figure 2. Both of these spectra show the standard characteristic modes of graphene, the D band (≈1350 cm−1), the G band (≈1580 cm−1) and the 2D band (≈2700 cm−1). The D band is not observed in pristine graphene due to symmetry effects, but it is observed when the Raman laser is focused close to an edge or is in the presence of basal plane defects [27]. The Raman D/G band intensity ratio (ID/IG) is an established method for estimating graphene crystallite size, with smaller ratios indicating better quality graphene films [28]. It is clear, from figure 2, that not only is the film uniformity measured and optically improved by the addition of copper to the nickel film, but the D band is also attenuated, resulting in improved graphene crystallite size. The utility of alloying copper with nickel for improving the graphene film quality is further demonstrated in figure 3, which plots the intensity ratio of the Raman D and G bands (ID/IG) as a function of the Cu/Ni ratio. There is a substantial improvement in graphene crystallite size, as measured by the characteristic ID/IG, which is most pronounced when a Cu:Ni film thickness ratio of 1.66 is used. The proposed mechanism for obtaining graphene in this approach is illustrated schematically in figure 4(a). When the metal/SiC system is heated, the metals intermix and diffuse into the SiC, inducing a Kirkendall boundary motion. Within this highly intermixed top layer, the nickel atoms dissociate the Si-C bonds by forming nickel silicide clusters [29], which results in the release of free atomic carbon. The unreacted nickel, plus the copper (which does not form any stable silicides), effectively mediate the graphitization of the atomic carbon, leading to precipitation in the form of few-layer graphene at the interface between the SiC and the intermixed catalytic layer. Note that the expected carbon saturation in Ni and Cu is extremely low. In particular, it only takes a few ppm of carbon to saturate the Cu-C system and trigger precipitation, which is one of the reasons why Cu is a preferred catalyst for graphitization. However, our experiments have indicated that Cu alone is not able to graphitize epitaxial SiC, likely because copper silicidation is not thermodynamically favored. As a result, Ni is required to release the atomic carbon from the epitaxial SiC film. At the same time, Ni alone on SiC would tend to form microscopic silicide clusters, as previously indicated [23] and also confirmed by the optical image in figure 2(a). Therefore, we suggest that the Cu in the Ni-Cu alloy leads to a two-fold benefit, acting as a 1) dilution medium for Ni and as 2) an accelerator of the graphitization of the atomic carbon, leading to a more uniform and efficient
Figure 3. A plot of the graphene D/G band Raman intensity ratio (a measure of graphene quality) for different thicknesses of copper deposited on ≈7 nm of nickel. The use of a Ni-Cu alloy leads to a substantial improvement in graphene quality as compared to the use of Ni as only a graphitization catalyst.
graphitization. The behavior in figure 3 also indicates that there is an optimal Cu:Ni ratio, since the process rates of the carbon release through the nickel silicidation and the graphitization, with the addition of copper, need to be balanced. When the Cu:Ni thickness ratio in the catalyst is increased above 1.66, the larger error bars and higher ID/IG ratios suggest that the optimal balance is gradually lost. Given the complexity of the system, we are unable to determine whether this mechanism is an exclusively solid state process. Intermediate liquid phases are not excluded, given the fact that the melting point of NiSi2 is approximately 990 °C [29] and given the proximity of the Cu melting temperature (1083 °C). X-ray photoelectron spectroscopy (XPS) analysis of a sample after graphitization, but prior to the removal of the metal catalysts and byproducts, is reported in figure 4(b). The high-resolution spectrum of the carbon region shows a composition rich in graphenic C-C (284 eV) and carbidic C-Si bonds (282.5 eV), as well as additional peaks attributed to different states of oxidized carbon. Copper and nickel are also detected with the XPS at concentrations of less than 1%. The corresponding spectrum of the carbon region after the removal of the top intermixed catalytic layer through wet etching in order to expose the graphitized silicon carbide is shown in figure 4(c). The predominant carbon bonds found after removal of the catalytic layer are the graphenic C-C (25%) and the carbidic C-Si (52%). The strong C-Si intensity after the etch, from the silicon carbide underneath the graphene layer, indicates the few-layer nature of the graphene and the presence of the graphene directly on the SiC surface, whilst the elemental analysis confirms a complete removal of nickel and copper. We did observe minor oxidation, and minute amounts of nitrogen, introduced from the etch process (containing nitric acid, see supporting information). Both Raman spectroscopy, and the presence of the graphenic C-C XPS bands after the catalyst and the byproduct removal, confirm that the graphene film grows directly on the 4
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Figure 4. (a) The graphitization of SiC proceeds through (i) ‘Kirkendall’ diffusion of the nickel and copper, (ii) silicide formation and release
of atomic carbon, leading to graphitization at the interface and (iii) removal of the intermixed metal layer, exposing the graphene on the SiC. (b)The XPS carbon spectrum of the reacted catalyst layer shows a complex convolution of a C-Si peak from the silicon carbide at 282.5 eV (20%), a C-C peak from grapheme, plus any other carbon material in the reacted layer at 284 eV (26%), and a high percentage of oxidized carbon and carbide species at higher binding energies (supporting info). (c) The XPS carbon spectrum, after the wet etching of the intermixed metal layer, shows a neater predominance of silicon carbide (31%) and graphene bonds (25%). The SiC signal is observed through the thin graphene layer.
graphene, is about 7 × 103 Ω/square. The latter measurement corresponds to a direct current resistivity of approximately 2 × 10−3 Ω m, as already reported for our unintentionally ntype doped epitaxial SiC on silicon [26], whereas the graphitized SiC sample would indicate an ‘effective resistivity’ of around approximately 2 × 10−5 Ω m. Note that the sheet resistance of a similarly graphitized sample using a 1 μm thickness SiC yields about 200 Ω/square, a value very close to the 174 ± 15 Ω/square obtained from the samples with onequarter of the thickness of the SiC. Therefore, we can confidently conclude that the current flow is essentially confined to the low resistivity of the thin graphene layer on top of the SiC surface. The sheet resistance of the graphene we obtain here, by catalytic graphitization of the SiC films, compares well to the value measured on a device fabricated from transferred graphene, from solid phase growth, on copper methodology, which approaches 1200 Ω/square [30].
SiC surface and distinguishes our work from previous reports [12, 13, 23, 24]. After performing a 30 s Ar+ sputter etch insitu with our XPS analysis on the sample with the exposed graphitized carbide in figure 4(c), the graphenic peak at 284 eV completely disappeared, leaving only carbon with carbidic C-Si bonds visible in the XPS spectrum. The Ar+ sputter etch rate is about 1 nm min−1, as calibrated on an SiO2 reference; this further supports the few-layer nature of the graphene on the SiC. The sheet resistance of the silicon carbide, which was graphitized using the optimal Ni-Cu alloy ratio, indicates more than an order of magnitude reduction, as compared to the bare epitaxial SiC film. The average sheet resistance, which over six graphitized SiC samples fabricated using the bimetal catalyst illustrated in figure 2(b), is 174 Ω/square, with a standard deviation of 15 Ω/square. The sheet resistance measured on the same 250 nm thick SiC film, without 5
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demonstrate the site-selectivity of this approach, we present a demonstration of the wafer-level fabrication of graphitized silicon carbide microbeams, following the procedure illustrated in figure 1. Suspended single- (cantilevers) and double-clamped (bridges) microbeams are used as basic components in MEMS technologies for sensing [32] and energy harvesting applications [33]. The SiC on Si wafer is coated with photoresist, and the structures are defined. Excess SiC is selectively etched away to the silicon layer through selective HCl plasma etching, and the remaining photoresist is removed with O2 plasma. The bimetal catalyst layer is deposited on the entire wafer, as graphitization is selective to only the defined SiC regions, with no reaction occurring on the silicon-only regions. Graphitization is then carried out as described in the experimental section. Finally, isotropic silicon etching is performed using the dry Si etchant XeF2 to release the graphitized SiC structures from the silicon substrate. All of the processes are carried out at the wafer level, and as the graphene synthesis is site-selective, only one lithographic step is required. The resultant graphitized SiC bridges and cantilevers are shown in the micrographs and schematics in figure 5(a). The Raman spectrum measured on a graphitized SiC microcantilever is detailed in figure 5(b), with all the graphene fingerprint modes identified, along with the Raman modes around 800 and 900 cm−1 from the underlying SiC. The characteristics of the Raman D, G and 2D modes of the graphene spectrum fall within the statistical distribution of the Ni-Cu mediated growth, indicating that the wafer-level fabrication method demonstrated here does not affect the ultimate quality of the few-layer graphene. The evidence of the longitudinal optical mode of SiC at 972 cm−1 indicates a successful release of the structure from the Si substrate, which is otherwise obscured by the presence of the second-order Si Raman mode [34]. The Raman spectrum in figure 5(c) is taken from a region that lies between the microbeams, showing the expected presence of the silicon from the bulk substrate, whilst the characteristic modes of both the SiC and the graphene are not found, demonstrating the successful siteselective growth of graphene.
a
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Figure 5. (a) Optical image (center) of a silicon wafer carrying
several thousands of graphitized silicon carbide microbeams. The SEM micrographs (left and right) show the released cantilevers and bridges. Scale bars, left: 200 μm, right: 50 μm. (b) Raman spectrum of a cantilever, indicating good quality, few-layer graphene onto a released SiC support. (c) Raman spectrum of the region between the microbeams, showing the 520 cm−1 band of Si. The inset shows a magnification of the 1300–1800 cm−1 region, indicating the absence of graphene and hence demonstrating the selective nature of the graphitization on the SiC structures only.
The substantial reduction in electrical resistance that we demonstrate through our graphitization method indicates the plausibility of using the proposed process as a replacement for metallic films in structures for the electrical actuation/readout of SiC-based MEMS devices. Moreover, the nanocoating nature of the graphitization would ensure maximum ability for scaling down resonator thicknesses, while still maintaining unaltered conduction characteristics. Optimal electrical conductions for driving MEMS/NEMS devices would remain around a few Ω/square, as per the 30 nm thick gold film reported by Li et al [31]; hence, they are still substantially lower than the 170 Ω/square of our graphitized SiC. Nevertheless, Lee et al [25] have already demonstrated the feasibility for driving insulating SiN membranes through transferred few-layer graphene.
4. Conclusions We demonstrate a transfer-free, self-aligned synthesis of good quality few-layer graphene on silicon carbide microbeams through wafer-level silicon technology processes. In contrast to sublimation-based routes, the synthesis of graphene takes place at a temperature compatible with Si technology, thereby yielding a graphene coating with low defectivity and an ability to act as a conducting nanolayer on top of the SiC structures. Beyond the exemplified impact as a metal replacement for SiC-based MEMS devices, this process opens the door for large-scale fabrication of graphene microstructures and nanostructures for countless electronic, photonic, optomechanical and sensing applications. The growth on silicon substrates at a temperature compatible with that of Si device fabrication, in addition to the use of a well-established, large-
3.2. Self-aligned fabrication of graphitized SiC microbeams
We use a prepatterned thin film of monocrystalline silicon carbide on silicon (SiC on Si) to obtain transfer-free and siteselective graphene through catalytic graphitization. To 6
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scale fabrication platform, also opens the door to a seamless integration of graphene devices with current CMOS technologies in a ‘More-than-Moore’ strategy for integrated microsystems and nanosystems.
[11] Macháč P, Fidler T, Cichoň S and Mišková L 2012 Thin Solid Films 520 5215 [12] Juang Z-Y, Wu C-Y, Lo C-W, Chen W-Y, Huang C-F, Hwang J-C, Chen F-R, Leou K-C and Tsai C-H 2009 Carbon 47 2026 [13] Kang C Y, Fan L L, Chen S, Liu Z L, Xu P S and Zou C W 2012 Appl. Phys. Lett. 100 251604 [14] Iacopi F, Brock R E, Iacopi A, Hold L and Dauskardt R H 2013 Acta Mater. 61 6533 [15] Sutter P 2009 Nat. Mater. 8 171 [16] de Heer W A, Berger C, Ruan M, Sprinkle M, Li X, Hu Y, Zhang B, Hankinson J and Conrad E 2011 Proc. Natl. Acad. Sci. USA 108 16900 [17] Emtsev K V et al 2009 Nat. Mater. 8 203 [18] Berger C et al 2004 J. Phys. Chem. B 108 19912 [19] Fukidome H et al 2011 J. Mater. Chem. 21 17242 [20] Gupta B, Notarianni M, Mishra N, Shafiei M, Iacopi F and Motta N 2014 Carbon 68 563 [21] Aristov V Y et al 2010 Nano Lett. 10 992 [22] Ouerghi A et al 2010 Appl. Phys. Lett. 96 191910 [23] Woodworth A A and Stinespring C D 2010 Carbon 48 1999 [24] Delamoreanu A, Rabot C, Vallee C and Zenasni A 2014 Carbon 66 48 [25] Lee S, Adiga V P, Barton R A, van der Zande A M, Lee G-H, Ilic B R, Gondarenko A, Parpia J M, Craighead H G and Hone J 2013 Nano Lett. 13 4275 [26] Wang L, Dimitrijev S, Han J, Iacopi A, Hold L, Tanner P and Harrison H B 2011 Thin Solid Films 519 6443 [27] Ferrari A C 2007 Solid State Commun. 143 47 [28] Ferrari A C and Basko D M 2013 Nat. Nanotechnol. 8 235 [29] Tinani M, Mueller A, Gao Y, Irene E A, Hu Y Z and Tay S P 2001 J. Vac. Sci. Technol. B 19 376 [30] Sun Z, Yan Z, Yao J, Beitler E, Zhu Y and Tour J M 2010 Nature 468 549 [31] Li M, Tang H X and Roukes M L 2007 Nat. Nanotechnol. 2 114 [32] Waggoner P S and Craighead H G 2007 Lab Chip 7 1238 [33] Radousky H B and Liang H 2012 Nanotechnology 23 502001 [34] Iacopi F, Walker G, Wang L, Malesys L, Ma S, Cunning B V and Iacopi A 2013 Appl. Phys. Lett. 102 011908
Acknowledgements The authors would like to acknowledge funding support from the Australian National Fabrication Facility, the Queensland State Government and SPTS Technologies (San Jose, Ca). FI is a recipient of a Future Fellowship from the Australian Research Council (FT120100445).
References [1] Novoselov K S, Falko V I, Colombo L, Gellert P R, Schwab M G and Kim K 2012 Nature 490 192 [2] Bao Q and Loh K P 2012 ACS Nano 6 3677 [3] Weiss N O, Zhou H, Liao L, Liu Y, Jiang S, Huang Y and Duan X 2012 Adv. Mater. 24 5782 [4] Kang J, Hwang S, Kim J H, Kim M H, Ryu J, Seo S J, Hong B H, Kim M K and Choi J-B 2012 ACS Nano 6 5360 [5] Caldwell J D et al 2010 ACS Nano 4 1108 [6] Henry Huang X M, Zorman C A, Mehregany M and Roukes M L 2003 Nature 421 496 [7] Kermany A R, Brawley G, Mishra N, Sheridan E, Bowen W P and Iacopi F 2014 Appl. Phys. Lett. 104 081901 [8] Macháč P, Fidler T, Cichoň S and Jurka V 2013 J. Mater. Sci: Mater. Electron. 24 3793 [9] Yoneda T, Shibuya M, Mitsuhara K, Visikovskiy A, Hoshino Y and Kido Y 2010 Surf. Sci. 604 1509 [10] Escobedo-Cousin E, Vassilevski K, Hopf T, Wright N, O'Neill A, Horsfall A, Goss J and Cumpson P 2013 J. Appl. Phys. 113 114309
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My IOPscience
Graphitized silicon carbide microbeams: wafer-level, self-aligned graphene on silicon wafers
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 325301 (http://iopscience.iop.org/0957-4484/25/32/325301) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 130.130.211.200 This content was downloaded on 16/07/2017 at 21:13 Please note that terms and conditions apply.
You may also be interested in: Solid source growth of graphene with Ni–Cu catalysts: towards high quality in situ graphene on silicon Neeraj Mishra, John J Boeckl, Anton Tadich et al. A thin film approach for SiC-derived graphene as an on-chip electrode for supercapacitors Mohsin Ahmed, Mohamad Khawaja, Marco Notarianni et al. Combining graphene with silicon carbide: synthesis and properties – a review Ivan Shtepliuk, Volodymyr Khranovskyy and Rositsa Yakimova High-quality, single-layered epitaxial graphene fabricated on 6H-SiC (0001) by flash annealing in Pb atmosphere and mechanism T W Hu, X T Liu, F Ma et al. Review on mechanism of directly fabricating wafer-scale graphene on dielectric substrates by chemical vapor deposition Jing Ning, Dong Wang, Yang Chai et al. Tension assisted metal transfer of graphene for Schottky diodes onto wafer scale substrates Jooho Lee, Su Chan Lee, Yongsung Kim et al. Fabrication of a Schottky junction diode with direct growth graphene on silicon by a solid phase reaction Golap Kalita, Ryo Hirano, Muhammed E Ayhan et al. Low temperature metal free growth of graphene on insulating substrates by plasma assisted chemical vapor deposition R Muñoz, C Munuera, J I Martínez et al.
Nanotechnology Nanotechnology 25 (2014) 325301 (7pp)
doi:10.1088/0957-4484/25/32/325301
Graphitized silicon carbide microbeams: wafer-level, self-aligned graphene on silicon wafers Benjamin V Cunning1, Mohsin Ahmed1, Neeraj Mishra1, Atieh Ranjbar Kermany1, Barry Wood2 and Francesca Iacopi1 1
Queensland Micro- and Nanotechnology Centre, Griffith University, Nathan 4111, Queensland, Australia Centre for Microscopy and Microanalysis, The University of Queensland, St. Lucia 4072, Queensland, Australia
2
E-mail: f.iacopi@griffith.edu.au Received 10 March 2014, revised 10 April 2014 Accepted for publication 22 May 2014 Published 23 July 2014 Abstract
Currently proven methods that are used to obtain devices with high-quality graphene on silicon wafers involve the transfer of graphene flakes from a growth substrate, resulting in fundamental limitations for large-scale device fabrication. Moreover, the complex three-dimensional structures of interest for microelectromechanical and nanoelectromechanical systems are hardly compatible with such transfer processes. Here, we introduce a methodology for obtaining thousands of microbeams, made of graphitized silicon carbide on silicon, through a site-selective and wafer-scale approach. A Ni-Cu alloy catalyst mediates a self-aligned graphitization on prepatterned SiC microstructures at a temperature that is compatible with silicon technologies. The graphene nanocoating leads to a dramatically enhanced electrical conductivity, which elevates this approach to an ideal method for the replacement of conductive metal films in silicon carbide-based MEMS and NEMS devices. S Online supplementary data available from stacks.iop.org/NANO/25/325301/mmedia Keywords: graphene, silicon carbide, self-aligned fabrication, MEMS (Some figures may appear in colour only in the online journal) 1. Introduction
encountered in microelectromechanical and nanoelectromechanical systems (MEMS & NEMS) unrealistic. Epitaxial silicon carbide (SiC) films on silicon (Si) are an ideal platform for the integration of graphene-coated structures for MEMS or NEMS applications, since crystalline SiC has outstanding mechanical properties [6, 7] and is a promising template for graphene growth through solid carbon source processes [8–13]. As patterning of the SiC on Si can be carried out through well-established photolithography and etching processes [14], the synergy of these properties allows for well-defined graphitized structures when using a selfaligned approach with critical dimensions that potentially reach down to a few nanometers. Currently, the most realistic route toward transfer-free, device-quality graphene on prepatterned solid carbon sources is based on silicon sublimation from SiC bulk wafers at very
Graphene is heralded as one of the most promising nanomaterials for a variety of applications, due to the extraordinary physical, optical, electronic and mechanical properties of its pristine form [1–3]. However, the massive impact that is expected from the utilization of such properties in actual microdevices or nanodevices at an extensive scale is still far from being a reality, due to hurdles in their replication in well-defined structures and to a substantial variability of device properties when using transferred graphene [4, 5]. Furthermore, the reliance on the manipulation of exfoliated or grown graphene flakes to realize devices does not allow for precisely defined positioning and dimensions, thereby making graphene integration with the complex structures commonly 0957-4484/14/325301+07$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
Nanotechnology 25 (2014) 325301
B V Cunning
Figure 1. Sequential steps for the wafer-level fabrication of graphitized silicon carbide microbeams on a silicon substrate. Once the SiC is
patterned, the few-layer graphene is grown selectively on the SiC structures via metal-mediated graphitization. The reacted metal layer is subsequently removed, and the structures are released from the substrate to form suspended beams.
Ni/SiC interface [10, 11], while the majority of people observed the graphene on the Ni surface [12, 13, 23, 24]. The latter case still clearly requires manual transfer of the graphene onto a semiconductor or an insulating surface to obtain a functional device, which hinders its utility for large-scale device fabrication. Herein, we build on the initial efforts of nickel mediated catalytic graphitization from SiC and show substantial advances through the use of epitaxial SiC on Si. By using a Ni-Cu alloy as opposed to a pure nickel catalyst, we achieve graphene growth directly on the SiC surface, thus eliminating the need for a manual graphene transfer, while concurrently improving the graphene crystallite size. We subsequently demonstrate a simple wafer-scale fabrication procedure, which validates the site-selective nature of the graphene growth and simultaneously realizes thousands of suspended SiC microstructures with a self-aligned nanocoating of fewlayer graphene. Electrical measurements of the obtained
high temperatures (1300–1600 °C) [15–18]. The prohibitive cost of the SiC wafers, as well as their patterning and micromachining, is a major limitation [1]. Attempts to replicate the success of this methodology through sublimation from a more affordable epitaxial SiC on silicon wafers have invariably shown substantially inferior graphene quality [19–22]. An alternative method for obtaining graphene from SiC involves a nickel-mediated catalytic graphitization at the SiC surface [9–12]. This approach takes place at temperatures ranging from 750 °C to 1200 °C, which is much lower than the sublimation process. However, the approach is not without its disadvantages. Earlier attempts at nickel-mediated graphitization from amorphous or crystalline SiC films on silicon have shown promise; however, the resulting graphene quality varies considerably. Additionally, there are conflicting reports regarding the location where the graphene was found. A few reports indicate that the graphene was obtained at the 2
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Figure 2. (a) The optical image after graphene growth on the SiC with the conventional single metal approach (Ni) shows large-scale non-
uniformity. The Raman spectrum from one of the bright regions, indicated by the circle, indicates high graphene defectivity (high D/G band ratio). (b) Optical image and typical Raman spectrum after graphene growth on the SiC using a Ni-Cu alloy in the form of a bilayer catalyst. Substantial improvements in both defectivity and uniformity are shown. The 2D/G Raman band ratio indicates bilayer graphene, whilst the D/G ratio of 0.5 reveals significantly lower defectivity when compared to figure 2(a).
thickness of ≈7 nm Ni and ≈15, 30 or 45 nm Cu, which was measured using a DecTak stylus profiler. For graphitization, the wafer was transferred into a Carbolite tube furnace and evacuated to a pressure below 1 × 10−3 mbar. The wafer was heated and maintained at 1050 °C for one hour and left to cool to room temperature under vacuum. The metal, metal silicides and carbon formed on the metal surface were all removed via sonication in a Freckle’s etch solution (70:10:5:5:10–85% H3PO4: Glacial acetic acid: 70% HNO3: 50% HBF4:H2O).
graphene point to our graphitization method as an ideal approach for the replacement of conducting layers in MEMS devices, in which metal films may degrade mechanical behavior, such as in quality factors for resonators [25].
2. Experimental 2.1. Graphitization
Epitaxial monocrystalline 3C-SiC layers, 250 nm thick, were grown onto two-inch Si(111) substrates in a custom-made, horizontal, hot wall, low-pressure chemical vapour furnace. The SiC films were grown at 1000 °C via an alternate supply of SiH4 and C3H6 and resulted in unintentionally n-type doped films, with a doping level around 1016–1017 at cm−3, as earlier described [26]. Thin layers of first nickel (99.95%), followed by copper (99.95%), were deposited using a DC Ar+ ion sputterer with a deposition current of 100 mA at a base pressure of 5 × 10−2 mbar. Nickel was sputtered for 10 s, and copper was sputtered at either 10, 20 or 30 s, yielding a
2.2. SiC patterning
The fabrication of graphitized microbeams proceeded through a self-aligned graphitization onto prepatterned SiC structures following the flow illustrated in figure 1. Conventional proximity photolithography with an AZ 6612 photoresist was used to define the regions of SiC to be etched in order to generate the structures. HCl plasma was used to selectively etch the SiC, and the remaining photoresist was removed with an O2 plasma etch. Graphitization then 3
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proceeded as per 2.1. The release of the SiC microstructures from the bulk Si substrate was achieved through a dry etch with XeF2, which etched anisotropically underneath the patterned SiC [7].
3. Results and discussion 3.1. Graphene synthesis and characterization
Raman spectra and corresponding optical images for films prepared with both nickel only and with the nickel/copper bilayer (1.66 Cu:Ni thickness ratio) are compared in figure 2. Both of these spectra show the standard characteristic modes of graphene, the D band (≈1350 cm−1), the G band (≈1580 cm−1) and the 2D band (≈2700 cm−1). The D band is not observed in pristine graphene due to symmetry effects, but it is observed when the Raman laser is focused close to an edge or is in the presence of basal plane defects [27]. The Raman D/G band intensity ratio (ID/IG) is an established method for estimating graphene crystallite size, with smaller ratios indicating better quality graphene films [28]. It is clear, from figure 2, that not only is the film uniformity measured and optically improved by the addition of copper to the nickel film, but the D band is also attenuated, resulting in improved graphene crystallite size. The utility of alloying copper with nickel for improving the graphene film quality is further demonstrated in figure 3, which plots the intensity ratio of the Raman D and G bands (ID/IG) as a function of the Cu/Ni ratio. There is a substantial improvement in graphene crystallite size, as measured by the characteristic ID/IG, which is most pronounced when a Cu:Ni film thickness ratio of 1.66 is used. The proposed mechanism for obtaining graphene in this approach is illustrated schematically in figure 4(a). When the metal/SiC system is heated, the metals intermix and diffuse into the SiC, inducing a Kirkendall boundary motion. Within this highly intermixed top layer, the nickel atoms dissociate the Si-C bonds by forming nickel silicide clusters [29], which results in the release of free atomic carbon. The unreacted nickel, plus the copper (which does not form any stable silicides), effectively mediate the graphitization of the atomic carbon, leading to precipitation in the form of few-layer graphene at the interface between the SiC and the intermixed catalytic layer. Note that the expected carbon saturation in Ni and Cu is extremely low. In particular, it only takes a few ppm of carbon to saturate the Cu-C system and trigger precipitation, which is one of the reasons why Cu is a preferred catalyst for graphitization. However, our experiments have indicated that Cu alone is not able to graphitize epitaxial SiC, likely because copper silicidation is not thermodynamically favored. As a result, Ni is required to release the atomic carbon from the epitaxial SiC film. At the same time, Ni alone on SiC would tend to form microscopic silicide clusters, as previously indicated [23] and also confirmed by the optical image in figure 2(a). Therefore, we suggest that the Cu in the Ni-Cu alloy leads to a two-fold benefit, acting as a 1) dilution medium for Ni and as 2) an accelerator of the graphitization of the atomic carbon, leading to a more uniform and efficient
Figure 3. A plot of the graphene D/G band Raman intensity ratio (a measure of graphene quality) for different thicknesses of copper deposited on ≈7 nm of nickel. The use of a Ni-Cu alloy leads to a substantial improvement in graphene quality as compared to the use of Ni as only a graphitization catalyst.
graphitization. The behavior in figure 3 also indicates that there is an optimal Cu:Ni ratio, since the process rates of the carbon release through the nickel silicidation and the graphitization, with the addition of copper, need to be balanced. When the Cu:Ni thickness ratio in the catalyst is increased above 1.66, the larger error bars and higher ID/IG ratios suggest that the optimal balance is gradually lost. Given the complexity of the system, we are unable to determine whether this mechanism is an exclusively solid state process. Intermediate liquid phases are not excluded, given the fact that the melting point of NiSi2 is approximately 990 °C [29] and given the proximity of the Cu melting temperature (1083 °C). X-ray photoelectron spectroscopy (XPS) analysis of a sample after graphitization, but prior to the removal of the metal catalysts and byproducts, is reported in figure 4(b). The high-resolution spectrum of the carbon region shows a composition rich in graphenic C-C (284 eV) and carbidic C-Si bonds (282.5 eV), as well as additional peaks attributed to different states of oxidized carbon. Copper and nickel are also detected with the XPS at concentrations of less than 1%. The corresponding spectrum of the carbon region after the removal of the top intermixed catalytic layer through wet etching in order to expose the graphitized silicon carbide is shown in figure 4(c). The predominant carbon bonds found after removal of the catalytic layer are the graphenic C-C (25%) and the carbidic C-Si (52%). The strong C-Si intensity after the etch, from the silicon carbide underneath the graphene layer, indicates the few-layer nature of the graphene and the presence of the graphene directly on the SiC surface, whilst the elemental analysis confirms a complete removal of nickel and copper. We did observe minor oxidation, and minute amounts of nitrogen, introduced from the etch process (containing nitric acid, see supporting information). Both Raman spectroscopy, and the presence of the graphenic C-C XPS bands after the catalyst and the byproduct removal, confirm that the graphene film grows directly on the 4
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Figure 4. (a) The graphitization of SiC proceeds through (i) ‘Kirkendall’ diffusion of the nickel and copper, (ii) silicide formation and release
of atomic carbon, leading to graphitization at the interface and (iii) removal of the intermixed metal layer, exposing the graphene on the SiC. (b)The XPS carbon spectrum of the reacted catalyst layer shows a complex convolution of a C-Si peak from the silicon carbide at 282.5 eV (20%), a C-C peak from grapheme, plus any other carbon material in the reacted layer at 284 eV (26%), and a high percentage of oxidized carbon and carbide species at higher binding energies (supporting info). (c) The XPS carbon spectrum, after the wet etching of the intermixed metal layer, shows a neater predominance of silicon carbide (31%) and graphene bonds (25%). The SiC signal is observed through the thin graphene layer.
graphene, is about 7 × 103 Ω/square. The latter measurement corresponds to a direct current resistivity of approximately 2 × 10−3 Ω m, as already reported for our unintentionally ntype doped epitaxial SiC on silicon [26], whereas the graphitized SiC sample would indicate an ‘effective resistivity’ of around approximately 2 × 10−5 Ω m. Note that the sheet resistance of a similarly graphitized sample using a 1 μm thickness SiC yields about 200 Ω/square, a value very close to the 174 ± 15 Ω/square obtained from the samples with onequarter of the thickness of the SiC. Therefore, we can confidently conclude that the current flow is essentially confined to the low resistivity of the thin graphene layer on top of the SiC surface. The sheet resistance of the graphene we obtain here, by catalytic graphitization of the SiC films, compares well to the value measured on a device fabricated from transferred graphene, from solid phase growth, on copper methodology, which approaches 1200 Ω/square [30].
SiC surface and distinguishes our work from previous reports [12, 13, 23, 24]. After performing a 30 s Ar+ sputter etch insitu with our XPS analysis on the sample with the exposed graphitized carbide in figure 4(c), the graphenic peak at 284 eV completely disappeared, leaving only carbon with carbidic C-Si bonds visible in the XPS spectrum. The Ar+ sputter etch rate is about 1 nm min−1, as calibrated on an SiO2 reference; this further supports the few-layer nature of the graphene on the SiC. The sheet resistance of the silicon carbide, which was graphitized using the optimal Ni-Cu alloy ratio, indicates more than an order of magnitude reduction, as compared to the bare epitaxial SiC film. The average sheet resistance, which over six graphitized SiC samples fabricated using the bimetal catalyst illustrated in figure 2(b), is 174 Ω/square, with a standard deviation of 15 Ω/square. The sheet resistance measured on the same 250 nm thick SiC film, without 5
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demonstrate the site-selectivity of this approach, we present a demonstration of the wafer-level fabrication of graphitized silicon carbide microbeams, following the procedure illustrated in figure 1. Suspended single- (cantilevers) and double-clamped (bridges) microbeams are used as basic components in MEMS technologies for sensing [32] and energy harvesting applications [33]. The SiC on Si wafer is coated with photoresist, and the structures are defined. Excess SiC is selectively etched away to the silicon layer through selective HCl plasma etching, and the remaining photoresist is removed with O2 plasma. The bimetal catalyst layer is deposited on the entire wafer, as graphitization is selective to only the defined SiC regions, with no reaction occurring on the silicon-only regions. Graphitization is then carried out as described in the experimental section. Finally, isotropic silicon etching is performed using the dry Si etchant XeF2 to release the graphitized SiC structures from the silicon substrate. All of the processes are carried out at the wafer level, and as the graphene synthesis is site-selective, only one lithographic step is required. The resultant graphitized SiC bridges and cantilevers are shown in the micrographs and schematics in figure 5(a). The Raman spectrum measured on a graphitized SiC microcantilever is detailed in figure 5(b), with all the graphene fingerprint modes identified, along with the Raman modes around 800 and 900 cm−1 from the underlying SiC. The characteristics of the Raman D, G and 2D modes of the graphene spectrum fall within the statistical distribution of the Ni-Cu mediated growth, indicating that the wafer-level fabrication method demonstrated here does not affect the ultimate quality of the few-layer graphene. The evidence of the longitudinal optical mode of SiC at 972 cm−1 indicates a successful release of the structure from the Si substrate, which is otherwise obscured by the presence of the second-order Si Raman mode [34]. The Raman spectrum in figure 5(c) is taken from a region that lies between the microbeams, showing the expected presence of the silicon from the bulk substrate, whilst the characteristic modes of both the SiC and the graphene are not found, demonstrating the successful siteselective growth of graphene.
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Figure 5. (a) Optical image (center) of a silicon wafer carrying
several thousands of graphitized silicon carbide microbeams. The SEM micrographs (left and right) show the released cantilevers and bridges. Scale bars, left: 200 μm, right: 50 μm. (b) Raman spectrum of a cantilever, indicating good quality, few-layer graphene onto a released SiC support. (c) Raman spectrum of the region between the microbeams, showing the 520 cm−1 band of Si. The inset shows a magnification of the 1300–1800 cm−1 region, indicating the absence of graphene and hence demonstrating the selective nature of the graphitization on the SiC structures only.
The substantial reduction in electrical resistance that we demonstrate through our graphitization method indicates the plausibility of using the proposed process as a replacement for metallic films in structures for the electrical actuation/readout of SiC-based MEMS devices. Moreover, the nanocoating nature of the graphitization would ensure maximum ability for scaling down resonator thicknesses, while still maintaining unaltered conduction characteristics. Optimal electrical conductions for driving MEMS/NEMS devices would remain around a few Ω/square, as per the 30 nm thick gold film reported by Li et al [31]; hence, they are still substantially lower than the 170 Ω/square of our graphitized SiC. Nevertheless, Lee et al [25] have already demonstrated the feasibility for driving insulating SiN membranes through transferred few-layer graphene.
4. Conclusions We demonstrate a transfer-free, self-aligned synthesis of good quality few-layer graphene on silicon carbide microbeams through wafer-level silicon technology processes. In contrast to sublimation-based routes, the synthesis of graphene takes place at a temperature compatible with Si technology, thereby yielding a graphene coating with low defectivity and an ability to act as a conducting nanolayer on top of the SiC structures. Beyond the exemplified impact as a metal replacement for SiC-based MEMS devices, this process opens the door for large-scale fabrication of graphene microstructures and nanostructures for countless electronic, photonic, optomechanical and sensing applications. The growth on silicon substrates at a temperature compatible with that of Si device fabrication, in addition to the use of a well-established, large-
3.2. Self-aligned fabrication of graphitized SiC microbeams
We use a prepatterned thin film of monocrystalline silicon carbide on silicon (SiC on Si) to obtain transfer-free and siteselective graphene through catalytic graphitization. To 6
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scale fabrication platform, also opens the door to a seamless integration of graphene devices with current CMOS technologies in a ‘More-than-Moore’ strategy for integrated microsystems and nanosystems.
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Acknowledgements The authors would like to acknowledge funding support from the Australian National Fabrication Facility, the Queensland State Government and SPTS Technologies (San Jose, Ca). FI is a recipient of a Future Fellowship from the Australian Research Council (FT120100445).
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