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Extreme Ultraviolet-Induced Surface Modification of Self-Assembled Monolayers of Furoxans Han-Na Hwang,† Jung Sook Kim,‡ Jung Moo Heo,§ Joon Won Park,‡ Kwang-Jin Hwang,*,§ and Chan-Cuk Hwang*,† Beamline Research DiVision, Pohang Accelerator Laboratory (PAL), Pohang, Kyungbuk 790-784, Korea, Center for Integrated Molecular Systems, Department of Chemistry, DiVision of Molecular and Life Sciences, Pohang UniVersity of Science and Technology, Pohang, Gyungbuk 790-784, Korea, and Department of Chemical System Engineering, Jochiwon, Chungnam, 339-701, Korea ReceiVed: April 19, 2009; ReVised Manuscript ReceiVed: July 20, 2009
We report extreme ultraviolet (EUV)-induced cleavage of furoxan self-assembled monolayers (SAMs) using high-resolution photoemission spectroscopy (HRPES) and near-edge X-ray absorption fine structure (NEXAFS). When SAMs of 3-methyl-4-furoxancarbaldehyde (FCA) and 4-(4-formylphenyl)-3-phenylfuroxan (FBA) are exposed to soft X-ray or EUV, a N 1s core level peak, which originates from N(2) nitrogen bonded to two oxygens in a furoxan ring, disappears completely in HRPES and a new structure appears at about 286 eV in NEXAFS. This indicates that two NO molecules in a furoxan are released by the lights, which is accompanied by the generation of carbon triple bonds. EUV values of ∼750 and 391 mJ/cm2 are necessary for the reaction of the FCA and FBA SAMs, respectively, suggesting that the SAMs are very sensitive to shorter wavelength lights, and the sensitivity depends on the functional group constituting the SAMs. Introduction Ultraviolet lithography (UVL) has been a standard patterning tool for several decades. However, its ability to make nanopatterns is expected to be limited to a feature size of 32 nm.1 In order to overcome the limit, next generation lithography techniques such as nanoimprint lithography, electron beam lithography (EBL), extreme ultraviolet lithography (EUVL), and scanning probe lithography have been proposed.2-5 EUVL among them is considered to be the most leading postoptical lithography due to mass production. As the wavelength of light decreases in photolithography, its depth of focus also reduces. Therefore, ultrathin photoresist (PR) is required in order to make a pattern below 32 nm. Selfassembled monolayers (SAMs) can be good candidates for the resist layer for photolithography using shorter wavelength lights such as EUV or soft X-ray6,7 as well as EBL.8,9 They are chemically more stable than molecular nanolayers formed in ultrahigh vacuum10,11 and have a more well-organized structure than polymers. If their end groups are modified and prepatterned with the lights, one can make functional nanopatterns by attaching molecules with specific functional groups on the surface. Alkyne-terminated SAMs can be used for anchoring other molecules via Huisgen [2 + 3] cycloaddition with azide (click chemistry).12-16 Furoxan is known to undergo efficient fragmentation of two NO molecules with concomitant production of a triple bond upon UV irradiation in gas or solution phase.17-21 If the same reaction takes place with the shorter wavelength lights, one can generate nanopatterns of triple bonds * Corresponding authors. E-mail:
[email protected] (C.-C.H.),
[email protected] (K.-J.H.). Telephone: +82-54-279-1254 (C.-C.H), +82-41-860-2271 (K.-J.H.). Fax: +82-54-279-1599 (C.-C.H.), +82-41-8666940 (K.-J.H.). † Beamline Research Division, Pohang Accelerator Laboratory (PAL). ‡ Pohang University of Science and Technology. § Hongik University.
on a solid surface, for example, with EUVL. Then, functionalized nanopatterns can be generated on the prepatterned alkynes through click chemistry. In this work, we demonstrate that triple bonds can be generated on the SAMs of 3-methyle-4-furoxancarbaldehyde (FCA) and 4-(4-formylphenyl)-3-phenylfuroxan (FBA, furoxanbiphenylaldehyde) with EUV or soft X-ray. When SAMs are exposed to the lights, a N 1s core level peak coming from the nitrogen(2) bonded to two oxygens in a furoxan disappears completely in high-resolution photoemission spectroscopy (HRPES) and a new structure corresponding to a carbon-carbon triple bond appears at about 286 eV in near-edge X-ray absorption fine structure (NEXAFS). This indicates that two NO molecules are set free from a furoxan ring by the shorter wavelength light and carbon-carbon triple bonds are generated at the same time. EUV values of ∼750 and 391 mJ/cm2 are necessary for the reaction of the FCA and FBA SAMs, respectively, suggesting that SAMs are sensitive to the lights, and the sensitivity can be further improved by modifying functional groups constituting the SAM. Experimental Methods Preparation of Aminosilylated Substrates. The silane coupling agent, (3-aminopropyl)diethoxymethylsilane (APDES), was purchased from Gelest, Inc., purified through fractional distillation, and stored under nitrogen. All other chemicals for the surface reaction were reagent grade from Sigma-Aldrich. Silylation reactions were carried out in anhydrous solvents using Aldrich Sure/Seal bottles. All washing solvents for the substrates are of HPLC grade from Mallinckrodt Laboratory Chemicals. Polished prime Si(100) wafers (dopant, phosphorus; resistivity, 1.5-2.1 Ω cm) were purchased from MEMC Electronic Materials, Inc. UV-grade fused silica plates were purchased from CVI Laser Co. Deionized water (18 MΩ cm) was produced by passing distilled water through a Milli-Q system (Millipore S. A.). The silicon substrates which were sonicated in piranha
10.1021/jp903599f CCC: $40.75 2009 American Chemical Society Published on Web 08/13/2009
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SCHEME 1: Schematic of (a) FCA and (b) FBA SAMs Formation on a SiO2 Surface
solution (70% H2SO4:30% H2O2) for more than 60 min and then washed with deionized water. After being placed in a solution of water/concentrated ammonia/30% hydrogen peroxide (v/v 5:1:1) at 80 °C for 10 min, the wafers were immersed in a solution of deionized water/concentrated hydrochloric acid/30% hydrogen peroxide (v/v 6:1:1) at 80 °C for 10 min. The substrates were taken out of the solution, washed with deionized water, and dried under vacuum. The substrates were reacted with 0.1% (v/v) (3-aminopropyl)diethoxymethylsilane solution in toluene for 3 h. After silylation, the APDES-treated silicon wafers were washed with toluene and baked for 35 min at 110 °C. The wafers were immersed in toluene, toluene-methanol (1:1 (v/v)), and methanol in a sequential manner, and they were sonicated for 3 min in each washing solution. Finally, the aminosilylated substrates were dried under vacuum. Preparation of Furoxan SAMs. Furoxan SAMs on the silicate were prepared as illustrated in Scheme 1. Furoxans, 3-methyl-4-furoxancarbaldehyde (FCA) and 4-(4-formylphenyl)3-phenyl-furoxan (FBA, furoxan-biphenylaldehyde), were prepared according to the procedures described in the literature.20 The aminosilylated substrates were immersed in anhydrous ethanol (20 mL) containing FCA (20 mg) under a nitrogen atmosphere at 50 °C for 1 day (Scheme 1a). In a similar way, aminosilylated substrates were reacted with FBA under the same conditions (Scheme 1b). The substrates were then sonicated for 1 min in ethanol, dichloromethane, and methanol sequentially and, finally, dried under vacuum. Measurements. HRPES and NEXAFS experiments were performed at the 7B1 beamline at the Pohang Accelerator Laboratory (PAL).22 During the experiments, the base pressure of the experimental chamber was kept at 2 × 10-10 Torr. HRPES spectra were taken using a commercial electron analyzer (PHOIBOS 150, SPECS) at normal emission. Binding energies of the Au 4f core level and Fermi level of the bulk Au were used for calibrating binding energy. HRPES and NEXAFS spectra of the pristine SAMs were recorded at a new position every time so as not to include data from damaged SAMs. The NEXAFS spectra were obtained at the carbon K-edge by
measuring sample current (I), which is normalized to photodiode current (I0), with varying photon energy before and after light irradiation. The photon energy was calibrated using the wellknown π* peak of graphite. Results and Discussion Figure 1 shows the surface sensitive N 1s core level spectra of the FCA (panels a and b) and FBA SAMs (panels c and d) before and after irradiation taken with a photon energy of 470 eV. The dots are measured spectra, and the solid curves are fitting results. The N 1s spectra of the pristine FCA (Figure 1a) and FBA SAMs (Figure 1c) were fitted with three components using a Gaussian width of 1.8 eV and Lorentzian width of 250 meV. The binding energies of the N1, N2, and N3 components are 404.9, 401.2, and 399.9 eV, respectively.23 To find the origins of these components, we obtained the N 1s core level spectrum from a furoxan thiol/Au surface without the APDES layer. As shown in the inset of Figure 1c, only N1 and N2 components are identified without a N3 component at the same binding energies using the same fitting parameters. Therefore, the curves are fitted using three components in the fitting procedure. The N1 and N2 components are due to the N(2) nitrogen bonded to two oxygens (O-N-O) and N(5) bonded to one oxygen (CdN-O) in a furoxan ring. The N3 component is associated with the imines and remaining nitrogens of the unreacted APDES layer. When SAMs are exposed to a soft X-ray of 470 eV, the N1 component disappears completely and the N2 component decreases equally, indicating that two NO are released from a furoxan ring. Notice that the N2 component still remains after irradiation. In fact, the intensities of the N1 and N2 components are different from each other before irradiation, even though they should ideally have the same intensity. The intensity of the N2 component in panels b and d of Figure 1is almost the same as the difference between N1 and N2 in panels a and c of Figure 1. One may expect intermediate states as a possible reason, where the furoxan ring is stabilized in an
Self-Assembled Monolayers of Furoxans
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Figure 1. N 1s core level spectra of the FCA and FBA SAMs (a,c) before and (b,d) after irradiation by a soft X-ray of 470 eV.
open state (-O-NdC-CdN-O+) or more possibly furazan formation through deoxyegnation.17-21 The intermediate states are more popular for the furoxan thiol/Au surface as shown in the inset of Figure 1c. The alkane chains are known to be easily damaged by irradiation with electrons or photons such as X-rays. In ref 9, the C 1s peak became broad and decreased in intensity after irradiation. The C 1s core level spectra of SAMs in this work (not shown here) do not show considerable change in intensity and FWHM by EUV irradiation. The intensity is unchanged within 6%, and the FWHM is almost the same before and after the irradiation. The XPS data indicate that EUV irradiation in this work does not lead to significant changes in the alkane chains under the furoxans. To obtain direct evidence for the presence of carbon triple bonds after EUV or soft X-ray irradiation, we measured NEXAFS, a powerful tool for detecting specific bonds in molecules.24 Figure 2 shows carbon K-edge NEXAFS spectra before and after irradiation for the FCA (Figure 2a) and FBA SAMs (Figure 2b). The filled (A, b) and open (B, O) circles are spectra measured before and after irradiation, respectively. Because FBA-SAMs are much more sensitive than FCA-SAMs, it was hard to take the whole energy range within the short lifetime of the FBA-SAMs. As shown in panels a and b of Figure 2, NEXAFS spectra exhibit some changes near 286 eV after soft X-ray irradiation. The spectra of pristine SAMs (A) was subtracted from the spectra after irradiation (B), making the difference spectra (C). According to previous literature, the positive peak at 286 eV in the inset of Figure 2b originates from the carbon triple bonds generated by the soft X-ray,25-27 and the two negative peaks can be assigned to π1*(CdN) and π2*(CdN).24,28,29 It is well-explained that the CdN bonds in the furoxan ring disappear through the release of NO from the furoxan ring with the shorter wavelength light. The cleavage reaction of the SAMs with the light is presented in Scheme 2, where two NO molecules in a furoxan ring are set free by light irradiation, and a triple bond is formed at the same time. Other sites such as the SiO2 substrate or the amines are unaffected by the irradiation.
Figure 2. Carbon K-edge NEXAFS spectra before (b) and after (O) the irradiation for (a) FCA and (b) FBA SAMs.
Furoxan SAMs were exposed to several wavelengths, including 92.5, 150, 340, and 630 eV. We observed the same cleavage of furoxan with all wavelengths (not shown here), including 92.5 eV (13.4 nm), which is currently used in EUVL. No significant changes were found before and after the K-edges of the constituting atoms, C, N, and O, of the SAMs. This suggests that the reaction results mainly from secondary electrons generated by the lights.
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SCHEME 2: Schematic of a Cleavage Reaction Triggered by Soft X-ray or EUV Irradiation on (a) FCA and (b) FBA SAMs
Hwang et al. This suggests that one can make desired funtional nanopatterns with EUVL using furoxan with potential alkyne functionality. Although the sensitivity is still higher than polymer PRs, we observed a possibility that it can be further improved by modifing functional groups in furoxan derivatives, which forms SAMs. Acknowledgment. This work was supported by the System IC 2010 project of the Korea Ministry of Knowledge Economy. References and Notes
To obtain the exact dosage required for the cleavage reactions, we used the following formula. Total dose (J/cm2) ) photon energy × number of photons per time × dosing time/area of beam. The number of photons can be obtained by I ) i/eQ, where i is the current of the photodiode and Q is the quantum efficiency of photodiode. We measured the current of photodiode, beam size, and dosing time at 92.5 eV. The beam size was measured by taking current through a plate that can move in the vertical and horizontal directions. Here, we assumed that the beam has a rectangular shape, and the distance between two points, where the current drops to zero, was considered as the size in one axis. The dosing time is defined as the time needed to make the N1 peak completely disappear. As a result, we obtained the necessary dosages of ∼750 and 391 mJ/cm2 for the EUV-induced reaction of FCA and FBA SAMs, respectively, where the dosages are an average value from several measurements and have an error of about (50 mJ/cm2. The values are larger than those for typical polymer PR, which shows several tens of millijoules per centimeter squared,30,31 but they are very sensitive within SAMs.32-34 The dosage for the reaction of FBA is half of the value for FCA, showing a possibility for further improving the sensitivity by modifying functional groups of furoxans. The results are in good agrement with the mass spectroscopy data, where the desorption of two NOs is dominant in FBA, whereas one NO desorption is quite detectable in the mass spectrum of FCA.20 Because the reaction is likely to originate mainly from photoinduced secondary electrons, SAMs can be applied to EBL23 as well as photolithography using shorter wavelength lights such as EUV or X-ray. Therefore, the furoxan SAMs are expected to be applied to a variety of fields, including sensors, diagnosis of biomaterials, and biochips, in combination with click chemistry and EUVL. Conclusions In summary, we generated EUV- or soft X-ray-induced alkynes on FCA and FBA SAMs through NO fragmentation.
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