Effective Method for Micro-Patterning Arene-Terminated Monolayers

Jun 22, 2016 - Effective Method for Micro-Patterning Arene-Terminated. Monolayers on a Si(111) Electrode. Yoshinori Yamanoi,*,†. Tetsuhiro Kobayashi...
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Effective Method for Micro-Patterning Arene-Terminated Monolayers on a Si(111) Electrode Yoshinori Yamanoi,*,† Tetsuhiro Kobayashi,† Hiroaki Maeda,† Mariko Miyachi,† Masato Ara,‡ Hirokazu Tada,‡ and Hiroshi Nishihara*,† †

Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka, 560-8531, Japan

‡

S Supporting Information *

ABSTRACT: Microstructured electrodes are significant to modern electrochemistry. A representative aromatic group, 4ferrocenylphenyl one, was covalently bound to a micropatterned silicon electrode via the arylation of a hydrogen-terminated silicon(111) surface formed selectively on a Si wafer. Starting from a silicon(100)-on-insulator (SOI) wafer, the aromatic monolayer was attached sequentially by spin-coating a resist, electron beam lithography, Cr/Au deposition, lift-off, anisotropic etching with aqueous KOH solution, and Pd-catalyzed arylation. Cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS) are used to characterize the coupling reaction between 4-ferrocenyl group and silicon substrate, and to confirm performance of the final modified microsized electrode. These data show that this synthetic protocol gives chemically welldefined and robust functionalized monolayers on a silicon semiconducting surface with a small electrode.

1. INTRODUCTION Applying micropattern to electrodes is an important technology for the preparation of devices such as memories, sensors, and actuators.1,2 Of particular interest is the chemical modification of silicon surfaces via the deposition and patterning of organic layers. The use of silicon−carbon linked monolayers increases the stability of the devices and prevents the oxidation of silicon, which tends to degrade the electronic performance. Most of the methods for producing micro sized silicon electrodes are based on lithography, which with electron-beam (EB) induced patterning is one of the most useful.3 Although microsized monolayers on hydrogen-terminated silicon surfaces have been reported,4−6 their evaluations are difficult, and surface characterization is still needed due to the trace amount of molecules attached. Integrated microsized electrodes have been developed to overcome such problems. Electrochemical analysis can effectively determine the extent of molecular attachment on a surface and also confirm the quality of the modified surface. We have recently developed the strategy to stabilize silicon surface that seems to satisfy all these criteria. The strategy in this work was based on the palladium-catalyzed immobilization of aromatic compounds onto a hydrogenterminated Si(111) surface via silicon−aryl bond formation.7 The arylation of Si−H surface with aryl iodides resulted in a well-defined and highly stable monolayer on the silicon surface. This paper describes the fabrication process of areneterminated monolayers attached on a micropatterned silicon on insulator (SOI) substrate prepared by electron beam (EB) lithography. Au/Cr layer was deposited on the surface after the fabrication of resist patterns.8,9 Lift-off removed the non© 2016 American Chemical Society

irradiated area, and the exposed Si was etched with KOH solution. 4-Ferrocenylphenyl groups were covalently attached to the micropatterned Si(111) area in the presence of Pd catalyst and base to monitor the progress of this arylation reaction by electrochemical measurements.

2. EXPERIMENTAL SECTION 2.1. Materials and Chemicals. SOI wafer, prepared by SIMOX, was purchased from E & M Co., Ltd. (its specifications are listed in Table 1). Water was purified using PF-Housing MODEL-III (for patterned electrode preparation) or AUTOPURE WD500 (for arylation reaction) equipment. Positive EB resist (ZEP520A) was purchased from ZEON Corporation.10,11 Espacer 300Z, a conductive polymer used to coat the resist, was purchased from Showa Denko.12 2-Propanol, pentyl acetate, and N,N-dimethylacetamide were purchased from Wako Pure Chemical Industries and used as received. 1,4-Dioxane and N,N-diisopropylethylamine were distilled and purged

Table 1. Specifications of SOI upper silicon layer

SiO2 layer lower silicon layer

thickness orientation resistivity type (dopant) thickness thickness orientation resistivity type (dopant)

61 ± 2.5 nm (100) ± 10° 9−18 Ω cm P-type (boron) 137.5 ± 6.5 nm ca. 775 μm (100) ± 0.17° 9−18 Ω cm P-type (boron)

Received: April 9, 2016 Revised: May 24, 2016 Published: June 22, 2016 6825

DOI: 10.1021/acs.langmuir.6b01370 Langmuir 2016, 32, 6825−6829

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Figure 1. Top view of the patterned microsized electrode.

Figure 2. Sequence of process to create the patterned microsized hydrogen-terminated silicon electrode. Cross-sectional views of: (i) spin-coating the resist, (ii) electron beam (EB) lithography, (iii) Cr (5 nm) and Au (50 nm) mask deposition, (iv) lift-off, and (v) anisotropic etching with HF (1%) then KOH (50%). 24 h under a nitrogen atmosphere. It was then successively rinsed ultrasonically with 1,4-dioxane, ethanol, and ultrapure water (Milli Q) for 5 min. The modified electrode was dried in a stream of argon gas. 2.4. Surface Characterization. Scanning electron microscopy (SEM) was conducted with a HITACH SU6600 microscope. Electrochemical measurements were performed using a modified silicon wafer working electrode, a Pt wire counter electrode, and an Ag/Ag+ (an Ag wire immersed in a 10 mM AgClO4/0.1 M Bu4NClO4/CH3CN solution) reference electrode in a standard onecompartment cell at room temperature. Cyclic voltammetry was performed using an ALS 650DT electrochemical analyzer. The electrolyte solution was tetra(n-butyl) ammonium perchlorate in dichloromethane (TBAP, 1 M). All electrochemical measurements have been performed in the dark to avoid photoinduced electron transfer process. XPS data were recorded with an ULVAC-PHI PHI5000 VersaProbe and a monochromatic Al Kα anode. The experimental resolution was 0.1 eV. The adventitious carbon C 1s peak at 285.0 eV was used as the calibration peak for all spectra.

with nitrogen gas before use. 4-Iodophenylferrocene was synthesized according to a known method.13 2.2. Preparation of Micropatterned SOI Electrode. The SOI wafer was ultrasonically washed with acetone for 3 min, and with 2propanol for 5 min to remove surface contamination. The EB resist (ZEP520A) was spin-coated onto cleaned SOI wafer for 3 s at 500 rpm and for 120 s at 8000 rpm with MIKASA 1H-D7 spin coater. The coated wafer was baked at 180 °C for 5 min to give a resist thickness of 150 nm. The antistatic coating (Espacer 300Z) was then spin-coated for 3 s at 300 rpm and for 60 s at 3000 rpm. The coated wafer was baked on a hot plate at 100 °C for 2 min; the thickness of anticharging layer was about 20 nm. EB lithography was conducted with an ELIONIX ELS-3700 system after these processes. The lithographed wafer was rinsed with pure water to remove the antistatic agent. The wafer was then developed with pentyl acetate for 4 min, and immersed in 2-propanol for 1 min; it was rinsed with pure water and dried in a stream of nitrogen gas. Cr and Au were successively deposited on the wafer in a vacuum chamber. The top material was lifted off with N,Ndimethylacetamide; and the wafer was then rinsed with 2-propanol and dried in a stream of nitrogen gas. Hydrogen-termination was carried out by the immersing the patterned wafer in 1% hydrogen fluoride aqueous solution for 1 min and in 50% potassium hydroxide aqueous solution until the completion of hydrogen generation (ca. 50 min). The etched wafer was rinsed with pure water and dried in a stream of argon gas. Caution: Proper precautions must be used when handling HF. HF is extremely corrosive to human tissue with contact resulting in painful burns. Laboratory work with HF should be conducted in an efficient hood with protective gloves. 2.3. Immobilization of 4-Iodophenylferrocene on the Hydrogen-Terminated Micropatterned SOI Electrode. The freshly etched and hydrogen-terminated SOI electrode was immediately immersed in a mixture of 1,4-dioxane, Pd(P(t-Bu)3)2 (0.08 mM), (i-Pr)2EtN (0.3 M) and 4-iodophenylferrocene (8 mM) at 100 °C for

3. RESULTS AND DISCUSSION 3.1. Preparation of Hydrogen-Terminated Patterned Silicon Electrode. Figure 1 presents the schematic view of the micropatterned electrode and its line/space configuration. The electrode was patterned on the SOI substrate in a 4000 μm × 4000 μm area. The hydrogen-terminated Si(111) surface appeared by etching of the top silicon layer of the SOI electrode, which is commercially available only in (100) orientation. The thickness of the top silicon layer was 61 nm; thus the hydrogen-terminated Si(111) surface could be prepared only to a width of 74 nm. Therefore, to increase the peak intensity of the electrochemical measurement, it was necessary to lengthen the hydrogen-terminated Si(111) surface, 6826

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Figure 3. (a) SEM observation with 5 kV acceleration voltage of micropattern electrode. (b) Digital microscopic observation of magnified micropattern electrode. (c) High-resolution SEM image with 20 kV acceleration of micropattern electrode after KOH anisotropic etching. Tilt angle θ = 25°.

Figure 4. Immobilization of 4-ferrocenylphenyl groups on the microsized electrode. (a) Schematic of presentation. (b) High-resolution XPS in the Au 4f region (left, measured with a takeoff angle of 45°) and the Si 2p region (right, measured with a takeoff angle of 5°). (c) Cyclic voltammograms at different scan rates. Electrode area: 5.9 × 10−3 cm2. (d) Plot of the oxidation peak currents in (c) vs scan rate.

i.e., to increase the perimeter of the Au/Cr mask. It was also necessary to connect the hydrogen-terminated Si(111) surface to an electrochemical measurement system. These considerations led to the micropatterned design shown in Figure 1. The 4000 μm × 4000 μm square was divided into 64 similar 500 μm × 500 μm square patterns. Each of these smaller squares consisted of 125 parallel lines of width 1 μm with a 5 μm connecting wire as the perimeter. An Au conductor of 50 μm width and 7.5 mm length was introduced for connection to the measurement system. The patterning process is illustrated in Figure 2. All processes were performed in a clean room to avoid dust contamination.

The SOI wafer required an antistatic layer (Espacer 300Z) to prevent charge accumulation during EB lithography. This layer was applied thinly by high speed spinning. The electrode was based on ca. 140 nm thick SiO2 layer with a 60 nm coating. After lithography, a 5 nm layer of Cr and 50 nm layer of Au were added as the mask by EB evaporation. The very thin Cr film enhanced the adhesion of the Au layer to the underlying substrate. Only the mask with resist film was removed by lift-off from nonirradiated region. We confirmed the complete removal of resist film (ZEP520A, Espacer 300Z) by the observation of XPS, because there was no peak in the regions of Cl 2p (200 6827

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Langmuir eV) and S 2p (164 eV), which consist of the resist film (Figure S3). The surface morphology of the patterned Si was observed by SEM and an optical microscope (Figure 3). Figure 3a shows a low-magnification SEM image of the patterned surface to demonstrate its patterning over a large area. Figure 3b shows an optical microscopic image. The well-defined patterning of the 3 μm lines on the silicon surface are also shown at higher magnification. The images show a good coverage of precise micropatterns. The underlying exposed Si(100) regions were then etched anisotropically using KOH solution.14,15 Because the Si(111) plane is etched more slowly than the Si(100) plane in alkaline solution, KOH etching generated a hydrogen-terminated Si(111) surface.16,17 Figure 3c shows a high-resolution image of the substrate tilted at 25° after anisotropic etching; the hydrogen-terminated area is observed as the slightly brightened area. Chemical etching proceeded only in the Au-coated main area on the silicon surface, and the surface of the etched electrode had a 55° slope with respect to the surface.18 3.2. Immobilization of 4-Ferrocenylphenyl Groups on the Patterned Microsized Hydrogen-Terminated Silicon Electrode. To demonstrate a practical application of the patterned hydrogen-terminated silicon electrode, the Si−H groups were arylated to generate covalent silicon-molecule contacts. We selected the 4-ferrocenylphenyl group for immobilization on microsized electrode owing to ferrocene’s attractive electrochemical characteristics such as low oxidation potential and high electrochemical durability, and fast electron transfer rate which arise due to its π-conjugation. The micropatterned electrode was immersed in a 1,4-dioxane solution of 4-iodophenylferrocene, N,N-diisopropylethylamine, and Pd catalyst under argon (Figure 4a). The resulting sample was investigated by XPS to check the quality and reproducibility as well as the immobilization (Figure 4b). Au 4f signals, taken at 45° from the surface normal, showed a clear chemical contrast between the inside and outside of the pattern features (Figure S3). The intense signals acquired inside the patterns correspond to Au 4f7/2 and Au 4f5/2 signals,19 indicating that the patterning was retained during the transformation. To enhance surface sensitivity of the XPS measurements in the Si 2p region, the photoemission angle between the sample normal and the analyzer was set at 5°. The Si 2p spectral region shows two distinct features: overlapped 2p3/2 and 2p1/2 components at around 99.5 eV associated with elemental silicon (Si0) and a broad band at around 103 eV due to oxidized H-terminated silicon surface or underlying SiO2 film.20 The Si 2p peak centered at 99.5 eV could not be divided into two peaks attributed Si0 and Si−C groups. The C 1s core level spectrum had one component at 285.0 eV that was assigned to C−H group of 4-ferrocenylphenyl moiety and carbon contamination in the XPS chamber. The corresponding Fe peaks for the same sample were not observed at 707 eV at all, which is the component for Fe 2p2/3 of Fe(II) complexes, owing to the relatively small amount of ferrocene on the surface. As the width of hydrogen-termination is estimated as 74 nm (= i.e., 61 nm/sin 55°), the total area of hydrogenterminated silicon is calculated as follows:

The attachable amount of ferrocenyl compound in this area is very small. Therefore, the intensity of iron signal might become weaker than that of lower detection limit.21,22 We also prepared another patterned microsized electrode, which increased silicon surface area per unit area (Figure S4). Although XPS was measured after the immobilization of 4ferrocenylphenyl group on the microsized silicon surface, the Fe 2p peak was not observed at all. Electrochemical studies of the immobilized 4-ferrocenylphenyl groups further characterized the modified surface. The modified surface showed the expected chemically reversible one-electron oxidations corresponding to ferrocene-containing monocations. Figure 4c shows cyclic voltammograms recorded at different scan rates after immobilization. The redox response of the ferrocenyl moiety appeared clearly at around 0.2 V vs Ag/Ag+. We compared the results of CV peak positions between patterned and nonpatterned Si(111) surface. These are approximate with peak position (ca. 0.2 V vs Ag/Ag+).23 The obtained curves of modified micro sized electrode showed that the characteristic peak current values increased with increasing scan rate. The plot of oxidation current peak against scan rate within 0.025 to 0.5 V s−1 (Figure 4d) shows a linear relation and Epa−Epc values were small, less than 20 mV in all scan rates.24 These electrochemical responses indicated the chemical attachment of the 4-ferrocenylphenyl group on the silicon surface. The total amount of 4-ferrocenylphenyl group attached to the surface was determined by peak area of cyclic voltammetry. The results therefore suggest a 1.0 × 10−10 mol/cm2 surface coverage on the micropatterned electrode, which is close to the value of a pristine hydrogen-terminated Si(111) surface. This coverage is approximately 22% of that expected for a hexagonally close-packed full monolayer of ferrocene molecules. 4-Ferrocenylphenyl group was not chemically attached onto the Au surface through the Au−C(sp2) bond by this transformation. Therefore, the amount of ferrocenyl group on Au surface was negligibly small. Overall, the fabrication method of this work enables the assembly and patterning of monolayers directly onto silicon without an intervening SiO2 layer. Generally, the full-width at half-maximum for the immobilized ferrocenyl molecules during cyclic voltammetry was 0.130−0.155 V in many cases.25 The broadening of the redox peak may have been due to the structure of the electrode. The chemical environment of the hydrogen-terminated area near Au/Cr mask was different from that near SiO2. Therefore, the peak became broadening may have been due to the slight difference in the redox potentials.26

4. CONCLUSIONS A microsized hydrogen-terminated silicon electrode was fabricated and used to immobilize aromatic compounds through covalent grafting. To enhance its electrochemical performance, an SOI surface was patterned using a Au/Cr mask into a grid of squares, with each square having several lines across it. The structure and chemical composition of the micropatterned surface were confirmed by SEM, optical microscope, and XPS, which revealed a sharp and complete chemical contrast between the patterned features and the surrounding areas. Representative aromatic group, 4-ferrocenylphenyl one, were then attached directly onto the micropatterned hydrogen-terminated silicon surface via Pd-mediated arylation. The chemical modification accurately retained the patterning during the transformation. This simple method

(495 μm × 2 × 125 lines + 3 μm × 2 × 125 lines) × 0.074 μm × 64 squares = 5.9 × 10−3 cm 2 6828

DOI: 10.1021/acs.langmuir.6b01370 Langmuir 2016, 32, 6825−6829

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(9) Homma, T.; Sato, H.; Mori, K.; Osaka, T.; Shoji, S. Area-Selective Formation of Macropore Array by Anisotropic Electrochemical Etching on an n-Si(100) Surface in Aqueous HF Solution. J. Phys. Chem. B 2005, 109, 5724−5727. (10) Nishida, T.; Notomi, M.; Iga, R.; Tamamura, T. Quantum Wire Fabrication by E-Beam Lithography Using High Resolution and HighSensitivity E-Beam Resist ZEP-520. Jpn. J. Appl. Phys. 1992, 31, 4508− 4514. (11) Hosaka, Y.; Oyama, G. T.; Oshima, A.; Enomoto, S.; Washio, M.; Tagawa, S. Pulse Radiolysis Study on a Highly Sensitive Chlorinated Resist ZEP520A. J. Photopolym. Sci. Technol. 2013, 26, 745−750. (12) Showa Denko Home Page. http://www.sdk.co.jp/products/45/ 75/1292/detail.html (accessed May 11, 2016). (13) Winters, M. U.; Dahlstedt, E.; Blades, H. E.; Wilson, C. J.; Frampton, M. J.; Anderson, H. L.; Albinsson, B. Probing the Efficiency of Electron Transfer through Porphyrin-Based Molecular Wires. J. Am. Chem. Soc. 2007, 129, 4291−4297. (14) Allongue, P.; Costa-Kieling, V.; Gerischer, H. Etching of Silicon in NaOH Solutions II. Electrochemical Studies of n-Si(111) and (100) and Mechanism of the Dissolution. J. Electrochem. Soc. 1993, 140, 1018−1026. (15) Palik, E. D.; Faust, J. W., Jr.; Gray, H. F.; Greene, R. F. Study of the Etch-Stop Mechanism in Silicon. J. Electrochem. Soc. 1982, 129, 2051−2059. (16) Pietsch, G. J.; Chabal, Y. J.; Higashi, G. S. Infrared-absorption Spectroscopy of Si(100) and Si(111) Surfaces after Chemomechanical Polishing. J. Appl. Phys. 1995, 78, 1650−1658. (17) Seidel, H.; Csepregi, L.; Heuberger, A.; Baumgärtel, H. Anisotropic Etching of Crystalline Silicon in Alkaline Solutions. J. Electrochem. Soc. 1990, 137, 3612−3626. (18) Mimura, K.; Ara, M.; Tada, H. Preparation of Nanogap Electrodes of Silicon by Chemical Etching. Mol. Cryst. Liq. Cryst. 2007, 472, 63−67. (19) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G. E. Handbook of X-Ray Photoelectron Spectroscopy; Perkin−Elmer: Eden Prairie, MN, 1979. (20) Sieval, A. B.; Linke, R.; Zuilhof, H.; Sudhölter, E. J. R. HighQuality Alkyl Monolayers on Silicon Surfaces. Adv. Mater. 2000, 12, 1457−1460. (21) Zanoni, R.; Cattaruzza, F.; Coluzza, C.; Dalchiele, E. A.; Decker, F.; Di Santo, G.; Flamini, A.; Funari, L.; Marrani, A. G. An AFM, XPS and Electrochemical Study of Molecular Electroactive Monolayers Formed by Wet Chemistry Functionalization of H-terminated Si(100) with Vinylferrocene. Surf. Sci. 2005, 575, 260−272. (22) Woodbridge, C. M.; Pugmire, D. L.; Johnson, R. C.; Boag, N. M.; Langell, M. A. J. Phys. Chem. B 2000, 104, 3085−3093. (23) Maeda, H.; Sakamoto, R.; Nishihara, H. Electron Transport of Bis(terpyridine)iron(II) complex wire on a Semiconducting Electrode. J. Electroanal. Chem. 2016, DOI: 10.1016/j.jelechem.2016.04.027. (24) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley & Sons: New York, 2001; pp 589−593. (25) Sabapathy, R. C.; Bhattacharyya, S.; Leavy, M. C.; Cleland, W. E., Jr.; Hussey, C. L. Electrochemical and Spectroscopic Characterization of Self-Assembled Monolayers of Ferrocenylalkyl Compounds with Amide Linkages. Langmuir 1998, 14, 124−136. (26) Rowe, G. K.; Creager, S. E. Redox and Ion-pairing Thermodynamics in Self-assembled Monolayers. Langmuir 1991, 7, 2307−2312.

might be useful for the fabrication of high-density arrays of electrodes with surface modifications, and thus miniature labon-a-chip devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b01370. Figures S1 and S2: Structures of photoresist; Figure S3: XPS survey spectrum; Figures S4: Structure of another patterned electrode (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work was financially supported in part by CREST from JST, Tokyo Kasei Chemical Promotion Foundation, Nippon Sheet Glass Foundation for Materials Science and Engineering, Precise Measurement Technology Promotion Foundation, Grant-in-Aids for Scientific Research (C) (No. 15K05604), Scientific Research (S) (No. 26220801), and Scientific Research on Innovative Areas “Molecular Architectonics: Orchestration of Single Molecules for Novel Functions” (area 2509, Nos. 26110505, 26110506, 16H00957, 16H00958, and 25110012) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

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REFERENCES

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DOI: 10.1021/acs.langmuir.6b01370 Langmuir 2016, 32, 6825−6829