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Surface potential microscopy for organized molecular systems
Applied Surface Science 188 (2002) 403–410
Surface potential microscopy for organized molecular systems Hiroyuki Sugimuraa,*, Kazuyuki Hayashia, Nagahiro Saitoa, Nobuyuki Nakagirib, Osamu Takaia a
Department of Materials Processing Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan b Core Technology Center, Nikon Co., Shinagawa, Tokyo 140-8601, Japan Received 3 September 2001; accepted 18 September 2001
Abstract Surface potentials of organosilane self-assembled monolayers (SAMs) formed on silicon substrates were measured using Kelvin-probe force microscopy (KFM) employing a SAM formed from n-octadecyltrimethoxysilane [ODS: CH3(CH2)17Si(OCH3)3] as a reference. The reference ODS surface was prepared in a micrometer scale on each of the samples based on a photolithographic technique using vacuum ultra-violet light at 172 nm. Another SAM was prepared on the same sample surface from heptadecafluoro-1,1,2,2-tetrahydro-decyl-1-trimethoxysilane (fluoroalkylsilane with 17 fluorine atoms, FAS17), 3,3,3trifluoropropyltrimethoxysilane (fluoroalkylsilane with three fluorine atoms, FAS3), n-(6-aminohexyl)aminopropyltrimethoxysilane (AHAPS) or (chloromethyl)phenyltrimethoxysilane (CMPS). Potentials of the surfaces covered with FAS17-SAM, FAS3-SAM and CMPS-SAM became more negative than ODS–SAM, while the surface covered with AHAPS-SAM showed a more positive surface potential than the reference. The acquired potential contrasts of the regions covered with FAS17, FAS3, AHAPS and CMPS with reference to ODS were 180, 150, þ50 and 30 mV, respectively. These results almost agreed with potentials expected from the dipole moments of the corresponding precursor molecules estimated by ab initio molecular orbital calculation. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Surface potential; Organosilane self-assembled monolayer; Kelvin-probe force microscopy; MO calculation; Dipole moment; Photolithography
1. Introduction Organic thin films with well-organized molecular arrangements have been used to control physical and chemical properties of solid substrates. Molecular selfassembling and Langmuir–Blodgett techniques were successfully applied to the preparation of such films [1]. The techniques and the films are, furthermore, expected
*
Corresponding author. Tel.: þ81-52-789-2796; fax: þ81-52-789-3260. E-mail address: [email protected] (H. Sugimura).
to play key roles in future microelectronic devices based on organic molecules. Among various organized molecular systems, monolayers constructed through chemisorption of organosilane molecules are of special interest since that can be formed on silicon (Si) which is a crucial material in microelectronics. Due to hydrophobic and van der Waals interactions between the alkyl chains of the organosilane molecules, the molecules are spontaneously organized into a thin layer of monomolecular thickness in which the molecules are closely packed each other [1–3]. Such a film belongs to a class of organic film known as self-assembled monolayer (SAM).
0169-4332/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 9 5 8 - 8
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Scanning probe microscopes (SPM) have been successfully applied to characterize organic thin films. In particular, the SPM-based surface potential microscopy is a promising technique in order to investigate electric properties of the films in minute scales. The method is, thus, frequently used to map surface potential distributions of organic monomolecular films on metal or semiconductor substrates [4–11]. Since the surface potentials are governed by dipole moments of SAMs which are closely related to their chemical structures, packing densities, molecular orientations and arrangements, structures of the SAMs are expected to be clarified through the surface potential microscopy. In order to elucidate molecular-level structures of SAMs through their surface potentials, it is necessary to acquire the potentials accurately and to compare those with theoretical simulation. In this paper, we report surface potentials of organosilane SAMs terminated with methyl, fluoromethyl, amino and chloromethyl groups. Through a microlithographic technique employing vacuum ultra-violet (VUV) light, coplanar microstructures consisting of the methyl-terminated SAM and another SAM were fabricated. Each of the samples was imaged by Kelvin-probe force microscopy (KFM) in order to acquire a potential contrast between the surfaces terminated with methyl groups and with fluoromethyl, amino or chloromethyl groups. Furthermore, we estimated dipole moments of the precursor organosilane molecules by ab initio molecular orbital (MO) calculations in order to discuss relations between the surface potentials and the dipole moments.
2. Experimental SAMs were formed form five types of precursors, i.e., n-octadecyltrimethoxysilane [ODS: Tokyo Kasei Organic Chemicals, CH3(CH2)17Si(OCH3)3], heptadecafluoro-1,1,2,2-tetrahydro-decyl-1-trimethoxysilane [fluoroalkylsilane with 17 fluorine atoms, FAS17: Shinetsu Chemical, CF3(CF2)7(CH2)2Si(OCH3)3], 3,3,3trifluoropropyltrimethoxysilane [fluoroalkylsilane with three fluorine atoms, FAS3: Shin-etsu Chemical, CF3(CF2)2Si(OCH3)3], n-(6-aminohexyl)aminopropyltrimethoxysilane [AHAPS: Gelest, H2N(CH2)6NH(CH2)3Si(OCH3)3] and (chloromethyl)phenyltrimethoxysilane [CMPS, Gelest, ClCH2C6H4Si(OCH3)3].
Fig. 1 shows the chemical structures of these precursors and the corresponding SAMs. Samples were fabricated by the procedures as shown in the lower part of Fig. 1. First, a substrate Si (n-type, 4–6 O cm) was photochemically cleaned by a UV/ozone cleaning method [12]. Due to this cleaning, organic contaminations on the substrate was completely removed and, moreover, a thin oxide layer was grown on the substrate. The surface of this oxide layer was so hydrophilic showing a water contact angle of almost 08 and, was most likely terminated with hydroxyl (OH) groups. Onto the cleaned Si substrate, a SAM was formed from ODS by chemical vapor deposition (CVD). The substrate was placed together with a glass cup filled with 0.2 cm3 ODS liquid into a 65 cm3 TeflonTM container. The container was sealed with a cap and placed for 3 h in an oven maintained at 150 8C. Vaporized ODS molecules were fixed on the Si substrates by the condensation reaction between the methoxy groups of the precursor and the surface OH groups [12]. Next, the substrate covered with ODS–SAM was photolithographically micropatterned. Details of this micropatterning method were reported elsewhere [13,14], here we summarize it only briefly. The samples was irradiated for 20 min through a photomask with a VUV light (an excimer lamp, Ushio Electric, UER 20–172 V, l ¼ 172 nm and 10 mW/cm2, was used) under a reduced pressure of 10 Pa. This technique is based on the photoinduced decomposition of the organic molecules by direct photoexcitation and the subsequent oxidation of the decomposed organic molecules with activated oxygen species generated by VUV irradiation of atmospheric oxygen molecules. In the VUV-irradiated regions, the SAM was selectively removed and, consequently, the underlying Si oxide (SiO2) layer was exposed. Similarly to the photocleaned Si substrate, such a photochemically exposed oxide surface was also hydrophilic so as to show a water contact angle of almost 08. Hence, its surface was most likely to be terminated with OH groups. Finally, the VUV-patterned ODS–SAM sample was treated with a different precursor. Since the OH-terminated regions have an affinity to organosilane molecules, while the unirradiated ODS surface has no reactivity, the second SAM (SAM-2) consisting of FAS17, FAS3, AHAPS or CMPS area-selectively formed confining to the VUV-irradiated pattern. In the case of FAS3, FAS17
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Fig. 1. Chemical structures of SAMs and their precursors: (a) ODS–SAM; (b) FAS17-SAM; (c) FAS3-SAM; (d) AHAPS-SAM; (e) CMPSSAM, and the procedures for sample preparation.
and CMPS, the VUV-patterned sample was treated as similarly to the CVD-growth of ODS–SAM. However, in the case of AHAPS, a glass cup was filled with 0.1 cm3 organosilane liquid diluted with 0.7 cm3 toluene under dry N2 atmosphere and the container was kept at 100 8C. Subsequently the sample was sonicated for 20 min in dehydrate ethanol, dehydrate toluene, NaOH (1 mM) and HNO3 (1 mM) in that order. Finally, the sample was rinsed with Milli-Q water and was blown dry with a N2 gas stream. Unlikely to the other SAMs, AHAPS-SAM formation was not reproducible when it was conducted without the sonication in the organic solvents and in the ionic solutions. Since an amino group in the aminosilane molecule, i.e., –NH2 or –NH–, may form a hydrogen or ionic bond with a methoxysilane group (BBSiOH3) or its hydrolysis form (BBSiOH), respectively, in another aminosilane molecule, AHAPS molecules are thought to form aggregates and to be further adsorbed on the AHAPS-SAM surface.
In order to obtain surface potential differences between ODS–SAM and the other SAMs, the coplanar FAS17/ODS, FAS3/ODS, AHAPS/ODS and CMPS/ ODS microstructures were observed in air by a KFM (Seiko Instruments, SPA-300HV þ SPI-3800N) using a gold-coated Si cantilever. Its force constant, resonance frequency and Q-factor were 1.8 N/m, 27.53 kHz and approximately 180, respectively. The cantilever was vibrated at a frequency of 27.59 kHz. An a.c. voltage of 2 V at a frequency of 25 kHz was applied between the probe and sample. KFM images of the sample surface was acquired at a probe scan rate of 0.1 Hz. These parameters were determined on the basis of our previous research [15]. When the intermittent contact mode was used for topographic imaging, a Si cantilever with a force constant of 20 N/m was employed. Structures and dipole moments of ODS-, FAS3-, FAS17-, AHAPS- and CMPS-precursor molecules were estimated by ab initio MO calculations using
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Gaussian 98 [16]. The structures and energies were obtained in the hybrid density functional theory with D95 basis set; B3LYP/95D [17,18]. The global minimum energy points, i.e., equilibrium structure, were found by optimization of all structural parameters.
3. Results and discussion Fig. 2a shows a topographic AFM image, acquired in the intermittent contact mode, of a VUV-patterned ODS–SAM sample. There are 5 mm 25 mm rectangular features in the image. In these VUV-irradiated regions, the underlying SiO2 had been exposed. The
features are recessed 1 nm from the surrounding ODS–SAM. This height contrast is smaller than the thickness of ODS–SAM, i.e., ca. 2 nm estimated by ellipsometry. The two regions, i.e., ODS–SAM and SiO2, have quite different chemical properties, namely, the former being hydrophobic and the later being hydrophilic. Consequently, adhesion forces on the regions become so different [19]. Such a difference in adhesion may distort the topographic height difference. A KFM image, which was acquired not simultaneously with Fig. 2a, of the sample shows that the recessed, thus, SiO2 exposed, regions have a surface potential 25 mV higher than that of the surrounding region covered with ODS–SAM (Fig. 2b).
Fig. 2. Topographic and potential images of micropatterned ODS–SAM samples: (a) AFM; (b) KFM images of ODS/SiO2; (c) AFM; (d) KFM images of ODS/ODS.
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A VUV-patterned ODS–SAM was treated again with ODS. Topographic and potential images of this sample are shown in Fig. 2c and d. The topographic image indicates that the regions covered with the second ODS–SAM (SAM2) are slightly higher than the surface of the first ODS–SAM (SAM1). Its topographic contrast is ca. 0.2 nm. Even when all the organic parts in ODS–SAM are photochemically decomposed, there remains a monolayer of a siloxane (Si–O–Si) network at the monolayer’s bottom as illustrated in Fig. 1a. Its thickness was estimated to be 0.2–0.3 nm by ellipsometry. Hence, the origin of the topographic height in Fig. 2c is ascribable to the remained siloxane monolayer. Although a slight potential contrast exists between SAM1 and SAM2
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as shown in Fig. 2d, its potential contrast is only a few millivolts. This is probably due to the siloxane monolayer. Thus, we conclude that SAM1 and SAM2 are almost identical and the effect of the remained siloxane monolayer on the potential is very small. Fig. 3a–d shows KFM images of the FAS17/ODS, FAS3/ODS, AHAPS/ODS and CMPS/ODS coplanar microstructures, respectively. Rectangular features of 5 mm 25 mm correspond to the regions covered with a SAM other than ODS–SAM, while the surrounding area is covered with ODS–SAM. As clearly seen in Fig. 3a–d, the regions covered with FAS17-SAM, FAS3-SAM and CMPS-SAM seem to show lower surface potentials than ODS–SAM. On the contrary, the region covered with AHAPS-SAM, as shown in
Fig. 3. Potential images of: (a) FAS17/ODS; (b) FAS3/ODS; (c) AHAPS/ODS; (d) CMPS/ODS.
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Fig. 3c, possesses a higher surface potential than ODS–SAM. The potential contrasts of the regions covered with FAS17-SAM, FAS3-SAM, AHAPSSAM and CMPS-SAM with reference to ODS–SAM are ca. 180, 150, þ50 and 30 mV, respectively. The main advantage of ODS–SAM as a reference is its hydrophobicity. Since the amount of adsorbed water, which affects measured surface potentials significantly [20,21], on ODS–SAM is small, surface potentials are reliably measured. Here we discuss surface potential contrasts between ODS–SAM and the others. A surface potential of a SAM on a Si substrate is expressed by Eq. (1) VSAM ¼
ðfsubst ftip Þ m þ þa AeSAM e0 e
(1)
where fsubst and ftip are work functions of the Si substrate and the KFM tip, respectively, e the electric charge, m the net dipole moment directed normally to the substrate surface, A the area occupied by each molecule, and eSAM and e0 are the permittivity of the SAM and free space, respectively. Eq. (1) consists of three terms: the first is ðfsubst ftip Þ=e which
represents the contact potential difference between the Si substrate and KFM tip, the second is m/AeSAMe0, which represents the dipole moment of an organic thin film derived from Helmholtz equation, the third is a which corresponds to a potential generated by trapped charges in the SiO2 layer. The surface potential difference between the regions covered with ODS–SAM and another SAM is obtained by Eq. (2) VSAM VODS ¼
mSAM mODS ASAM eSAM e0 AODS eODS e0
(2)
where the first and third terms of Eq. (1) do not remain. We assumed that differences in A and e between ODS– SAM and the other SAMs are not so significant, since the SAMs are laterally connected with an identical Si–O–Si network as shown in Fig. 1 and are formed mainly from hydrocarbons. Thus, the potential contrast is considered to be primarily governed by the difference in dipole moment. Dipole moments of ODS, FAS17, FAS3, AHAPS and CMPS molecules were calculated and shown in Fig. 4. We employed a slightly simplified model for each molecule in which each of three methoxy groups
Fig. 4. Molecular structures and dipole moments of the model molecules for: (a) ODS; (b) FAS17; (c) FAS3; (d) AHAPS; (e) CMPS.
H. Sugimura et al. / Applied Surface Science 188 (2002) 403–410
is replaced with a hydrogen atom, since the methoxy groups in the precursor molecules do not remain in the SAMs. The modeled ODS, FAS17, FAS3, AHAPS and CMPS molecule have molecular lengths of 2.35, 1.34, 0.43, 1.43 and 0.62 nm and dipole moments of 2.35, 3.41, 2.91, 1.04 and 2.40 Debye, respectively. These dipole moments incline 26.5, 25.3, 3.6, 12.4 and 55.18, respectively, with respect to each of the molecular chains as shown in Fig. 4. Vertical components of the dipole moments for the model ODS and AHAPS molecules are positive, namely, the dipole moments are directing from the bottom to the top, while those for the other model molecules, i.e., FAS17, FAS3 and CMPS are negative due to electron negativities of fluorine and chlorine atoms existing in their head groups. The dipole moments of the model FAS17, FAS3, AHAPS and CMPS molecules were compared with that of the ODS model with regard to their vertical components. The estimated vertical dipole moment differences (Dm?) are as follows: Dm?ðFAS17ODSÞ ð3:92DÞ < Dm?ðFAS3ODSÞ ð3:71DÞ
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that ASAM and eSAM are identical for all the SAMs. One plausible reason for the discrepancy between the experimental and calculated results is the molecular orientation in SAM, since the above discussion is based on the assumption that molecules in a SAM are aligned perpendicular to the substrate surface. It is known that SAMs are frequently constructed from inclined molecules [1,22]. The difference in ASAM is also responsible. For example, FAS3-SAM incompletely covered a substrate compared with FAS17-SAM [20]. Since the molecular length of FAS3 is much shorter than that of FAS17, intermolecular interactions, which are necessary to form a densely packed monolayer, is so weak that FAS3-SAM is loosely packed. Further experimental results and theoretical considerations are certainly necessary in order to completely explain the surface potential contrasts between the SAMs, however, it can be concluded that the surface potential of an organosilane SAM is primarily governed by a dipole moment of the organosilane molecules consisting of the SAM.
< Dm?ðCMPSODSÞ ð1:85DÞ < Dm?ðODSODSÞ ð0DÞ < Dm?ðAHAPSODSÞ ðþ0:53DÞ
(3)
The order and signs of Dm? qualitatively agree with the order and signs of the surface potential contrasts (DV) as shown in Fig. 5. The line indicated in Fig. 5 is the relation derived form Eq. (2) with assuming
Fig. 5. Relation between Dm? and DV.
4. Conclusion Organosilane SAMs formed from ODS, FAS17, FAS3, AHAPS and CMPS have been studied by KFM and MO calculation. Surface potentials of these SAMs were measured with employing ODS–SAM as a reference. On each of the samples, this reference SAM and another SAM were microfabricated. Through this approach in which potentials of two SAMs are simultaneously measured and compared, potential contrasts between the SAM and the reference can be measured more accurately without unreproducibility which sometimes appears when measurements are conducted separately. These potential contrasts almost agreed with those predicted from the dipole moments of the corresponding precursor molecules estimated by MO calculation. Although the true dipole moment of a SAM is probably different from that of the precursor molecule due to interactions between the molecules, it was confirmed that the surface potential is primarily governed by the dipole moment of the precursor molecule. KFM will be a powerful tool for the study on structures of organic molecular monolayers, since surface potentials of the monolayers depend both on their chemical structures and packing densities.
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Acknowledgements This research has been supported by Grant-in-Aid for Scientific Research on Priority Area of ‘‘NanoMechanics of Atoms and Molecules’’ from the Ministry of Education, Culture, Sports, Science and Technology.
References [1] A. Ulman, An Introduction to Ultrathin Organic Films from Langmuir–Blodgett to Self-assembly, Academic Press, Boston, 1991. [2] E.P. Plueddemann, Silane Coupling Agents, 2nd Edition, Plenum Press, New York, 1991. [3] J. Sagiv, J. Am. Chem. Soc. 102 (1980) 92. [4] M. Fujihira, Annu. Rev. Mater. Sci. 29 (1999) 353. [5] S.D. Evans, A. Ulman, Chem. Phys. Lett. 170 (1990) 462. [6] H. Yokoyama, K. Saito, T. Inoue, Mol. Electron. Bioelectron. 3 (1992) 79. [7] M. Yasutake, D. Aoki, M. Fujihira, Thin Solid Films 242 (1994) 33. [8] T. Kajiyama, K. Khuwattanasil, A. Takahara, J. Vac. Sci. Technol. B 16 (1998) 121.
[9] J. Lu¨ , E. Delamatche, L. Eng, R. Bennewitz, E. Meyer, H.J. Gu¨ ntherodt, Langmuir 15 (1999) 8184. [10] K. Yagi, M. Fujihira, Appl. Surf. Sci. 157 (2000) 405. [11] H. Sugimura, K. Hayashi, N. Saito, O. Takai, N. Nakagiri, Jpn. J. Appl. Phys. 40 (2001) L174. [12] H. Sugimura, N. Nakagiri, J. Photopolym. Sci. Technol. 10 (1997) 661. [13] H. Sugimura, N. Nakagiri, Appl. Phys. A 66 (1998) S427. [14] H. Sugimura, K. Ushiyama, A. Hozumi, O. Takai, Langmuir 16 (2000) 885. [15] H. Sugimura, K. Hayashi, N. Saito, O. Takai, N. Nakagiri, Jpn. J. Appl. Phys. 40 (2001) 4373. [16] M.J. Frisch, et al., Gaussian 98, Revision A.9, Gaussian Inc., Pittsburgh, PA, 1998. [17] T.H. Dunning, P.J. Hay, in: H.F. Schaefer (Ed.), Modern Theoretical Chemistry, Vol. 3, Plenum Press, New York, 1976, p. 1. [18] A.D. Beck, J. Phys. Chem. 98 (1993) 5648. [19] H. Sugimura, K. Hayashi, Y. Amano, O. Takai, A. Hozumi, J. Vac. Sci. Technol. A 19 (2001) 1261. [20] H. Bluhm, T. Inoue, M. Salmeron, Surf. Sci. Lett. 462 (2000) 599. [21] S. Ono, M. Takeuchi, T. Takahashi, Appl. Phys. Lett. 78 (2001) 1086. [22] A. Hozumi, K. Ushiyama, H. Sugimura, O. Takai, Langmuir 15 (1999) 7600.
Surface potential microscopy for organized molecular systems Hiroyuki Sugimuraa,*, Kazuyuki Hayashia, Nagahiro Saitoa, Nobuyuki Nakagirib, Osamu Takaia a
Department of Materials Processing Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan b Core Technology Center, Nikon Co., Shinagawa, Tokyo 140-8601, Japan Received 3 September 2001; accepted 18 September 2001
Abstract Surface potentials of organosilane self-assembled monolayers (SAMs) formed on silicon substrates were measured using Kelvin-probe force microscopy (KFM) employing a SAM formed from n-octadecyltrimethoxysilane [ODS: CH3(CH2)17Si(OCH3)3] as a reference. The reference ODS surface was prepared in a micrometer scale on each of the samples based on a photolithographic technique using vacuum ultra-violet light at 172 nm. Another SAM was prepared on the same sample surface from heptadecafluoro-1,1,2,2-tetrahydro-decyl-1-trimethoxysilane (fluoroalkylsilane with 17 fluorine atoms, FAS17), 3,3,3trifluoropropyltrimethoxysilane (fluoroalkylsilane with three fluorine atoms, FAS3), n-(6-aminohexyl)aminopropyltrimethoxysilane (AHAPS) or (chloromethyl)phenyltrimethoxysilane (CMPS). Potentials of the surfaces covered with FAS17-SAM, FAS3-SAM and CMPS-SAM became more negative than ODS–SAM, while the surface covered with AHAPS-SAM showed a more positive surface potential than the reference. The acquired potential contrasts of the regions covered with FAS17, FAS3, AHAPS and CMPS with reference to ODS were 180, 150, þ50 and 30 mV, respectively. These results almost agreed with potentials expected from the dipole moments of the corresponding precursor molecules estimated by ab initio molecular orbital calculation. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Surface potential; Organosilane self-assembled monolayer; Kelvin-probe force microscopy; MO calculation; Dipole moment; Photolithography
1. Introduction Organic thin films with well-organized molecular arrangements have been used to control physical and chemical properties of solid substrates. Molecular selfassembling and Langmuir–Blodgett techniques were successfully applied to the preparation of such films [1]. The techniques and the films are, furthermore, expected
*
Corresponding author. Tel.: þ81-52-789-2796; fax: þ81-52-789-3260. E-mail address: [email protected] (H. Sugimura).
to play key roles in future microelectronic devices based on organic molecules. Among various organized molecular systems, monolayers constructed through chemisorption of organosilane molecules are of special interest since that can be formed on silicon (Si) which is a crucial material in microelectronics. Due to hydrophobic and van der Waals interactions between the alkyl chains of the organosilane molecules, the molecules are spontaneously organized into a thin layer of monomolecular thickness in which the molecules are closely packed each other [1–3]. Such a film belongs to a class of organic film known as self-assembled monolayer (SAM).
0169-4332/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 9 5 8 - 8
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Scanning probe microscopes (SPM) have been successfully applied to characterize organic thin films. In particular, the SPM-based surface potential microscopy is a promising technique in order to investigate electric properties of the films in minute scales. The method is, thus, frequently used to map surface potential distributions of organic monomolecular films on metal or semiconductor substrates [4–11]. Since the surface potentials are governed by dipole moments of SAMs which are closely related to their chemical structures, packing densities, molecular orientations and arrangements, structures of the SAMs are expected to be clarified through the surface potential microscopy. In order to elucidate molecular-level structures of SAMs through their surface potentials, it is necessary to acquire the potentials accurately and to compare those with theoretical simulation. In this paper, we report surface potentials of organosilane SAMs terminated with methyl, fluoromethyl, amino and chloromethyl groups. Through a microlithographic technique employing vacuum ultra-violet (VUV) light, coplanar microstructures consisting of the methyl-terminated SAM and another SAM were fabricated. Each of the samples was imaged by Kelvin-probe force microscopy (KFM) in order to acquire a potential contrast between the surfaces terminated with methyl groups and with fluoromethyl, amino or chloromethyl groups. Furthermore, we estimated dipole moments of the precursor organosilane molecules by ab initio molecular orbital (MO) calculations in order to discuss relations between the surface potentials and the dipole moments.
2. Experimental SAMs were formed form five types of precursors, i.e., n-octadecyltrimethoxysilane [ODS: Tokyo Kasei Organic Chemicals, CH3(CH2)17Si(OCH3)3], heptadecafluoro-1,1,2,2-tetrahydro-decyl-1-trimethoxysilane [fluoroalkylsilane with 17 fluorine atoms, FAS17: Shinetsu Chemical, CF3(CF2)7(CH2)2Si(OCH3)3], 3,3,3trifluoropropyltrimethoxysilane [fluoroalkylsilane with three fluorine atoms, FAS3: Shin-etsu Chemical, CF3(CF2)2Si(OCH3)3], n-(6-aminohexyl)aminopropyltrimethoxysilane [AHAPS: Gelest, H2N(CH2)6NH(CH2)3Si(OCH3)3] and (chloromethyl)phenyltrimethoxysilane [CMPS, Gelest, ClCH2C6H4Si(OCH3)3].
Fig. 1 shows the chemical structures of these precursors and the corresponding SAMs. Samples were fabricated by the procedures as shown in the lower part of Fig. 1. First, a substrate Si (n-type, 4–6 O cm) was photochemically cleaned by a UV/ozone cleaning method [12]. Due to this cleaning, organic contaminations on the substrate was completely removed and, moreover, a thin oxide layer was grown on the substrate. The surface of this oxide layer was so hydrophilic showing a water contact angle of almost 08 and, was most likely terminated with hydroxyl (OH) groups. Onto the cleaned Si substrate, a SAM was formed from ODS by chemical vapor deposition (CVD). The substrate was placed together with a glass cup filled with 0.2 cm3 ODS liquid into a 65 cm3 TeflonTM container. The container was sealed with a cap and placed for 3 h in an oven maintained at 150 8C. Vaporized ODS molecules were fixed on the Si substrates by the condensation reaction between the methoxy groups of the precursor and the surface OH groups [12]. Next, the substrate covered with ODS–SAM was photolithographically micropatterned. Details of this micropatterning method were reported elsewhere [13,14], here we summarize it only briefly. The samples was irradiated for 20 min through a photomask with a VUV light (an excimer lamp, Ushio Electric, UER 20–172 V, l ¼ 172 nm and 10 mW/cm2, was used) under a reduced pressure of 10 Pa. This technique is based on the photoinduced decomposition of the organic molecules by direct photoexcitation and the subsequent oxidation of the decomposed organic molecules with activated oxygen species generated by VUV irradiation of atmospheric oxygen molecules. In the VUV-irradiated regions, the SAM was selectively removed and, consequently, the underlying Si oxide (SiO2) layer was exposed. Similarly to the photocleaned Si substrate, such a photochemically exposed oxide surface was also hydrophilic so as to show a water contact angle of almost 08. Hence, its surface was most likely to be terminated with OH groups. Finally, the VUV-patterned ODS–SAM sample was treated with a different precursor. Since the OH-terminated regions have an affinity to organosilane molecules, while the unirradiated ODS surface has no reactivity, the second SAM (SAM-2) consisting of FAS17, FAS3, AHAPS or CMPS area-selectively formed confining to the VUV-irradiated pattern. In the case of FAS3, FAS17
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Fig. 1. Chemical structures of SAMs and their precursors: (a) ODS–SAM; (b) FAS17-SAM; (c) FAS3-SAM; (d) AHAPS-SAM; (e) CMPSSAM, and the procedures for sample preparation.
and CMPS, the VUV-patterned sample was treated as similarly to the CVD-growth of ODS–SAM. However, in the case of AHAPS, a glass cup was filled with 0.1 cm3 organosilane liquid diluted with 0.7 cm3 toluene under dry N2 atmosphere and the container was kept at 100 8C. Subsequently the sample was sonicated for 20 min in dehydrate ethanol, dehydrate toluene, NaOH (1 mM) and HNO3 (1 mM) in that order. Finally, the sample was rinsed with Milli-Q water and was blown dry with a N2 gas stream. Unlikely to the other SAMs, AHAPS-SAM formation was not reproducible when it was conducted without the sonication in the organic solvents and in the ionic solutions. Since an amino group in the aminosilane molecule, i.e., –NH2 or –NH–, may form a hydrogen or ionic bond with a methoxysilane group (BBSiOH3) or its hydrolysis form (BBSiOH), respectively, in another aminosilane molecule, AHAPS molecules are thought to form aggregates and to be further adsorbed on the AHAPS-SAM surface.
In order to obtain surface potential differences between ODS–SAM and the other SAMs, the coplanar FAS17/ODS, FAS3/ODS, AHAPS/ODS and CMPS/ ODS microstructures were observed in air by a KFM (Seiko Instruments, SPA-300HV þ SPI-3800N) using a gold-coated Si cantilever. Its force constant, resonance frequency and Q-factor were 1.8 N/m, 27.53 kHz and approximately 180, respectively. The cantilever was vibrated at a frequency of 27.59 kHz. An a.c. voltage of 2 V at a frequency of 25 kHz was applied between the probe and sample. KFM images of the sample surface was acquired at a probe scan rate of 0.1 Hz. These parameters were determined on the basis of our previous research [15]. When the intermittent contact mode was used for topographic imaging, a Si cantilever with a force constant of 20 N/m was employed. Structures and dipole moments of ODS-, FAS3-, FAS17-, AHAPS- and CMPS-precursor molecules were estimated by ab initio MO calculations using
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Gaussian 98 [16]. The structures and energies were obtained in the hybrid density functional theory with D95 basis set; B3LYP/95D [17,18]. The global minimum energy points, i.e., equilibrium structure, were found by optimization of all structural parameters.
3. Results and discussion Fig. 2a shows a topographic AFM image, acquired in the intermittent contact mode, of a VUV-patterned ODS–SAM sample. There are 5 mm 25 mm rectangular features in the image. In these VUV-irradiated regions, the underlying SiO2 had been exposed. The
features are recessed 1 nm from the surrounding ODS–SAM. This height contrast is smaller than the thickness of ODS–SAM, i.e., ca. 2 nm estimated by ellipsometry. The two regions, i.e., ODS–SAM and SiO2, have quite different chemical properties, namely, the former being hydrophobic and the later being hydrophilic. Consequently, adhesion forces on the regions become so different [19]. Such a difference in adhesion may distort the topographic height difference. A KFM image, which was acquired not simultaneously with Fig. 2a, of the sample shows that the recessed, thus, SiO2 exposed, regions have a surface potential 25 mV higher than that of the surrounding region covered with ODS–SAM (Fig. 2b).
Fig. 2. Topographic and potential images of micropatterned ODS–SAM samples: (a) AFM; (b) KFM images of ODS/SiO2; (c) AFM; (d) KFM images of ODS/ODS.
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A VUV-patterned ODS–SAM was treated again with ODS. Topographic and potential images of this sample are shown in Fig. 2c and d. The topographic image indicates that the regions covered with the second ODS–SAM (SAM2) are slightly higher than the surface of the first ODS–SAM (SAM1). Its topographic contrast is ca. 0.2 nm. Even when all the organic parts in ODS–SAM are photochemically decomposed, there remains a monolayer of a siloxane (Si–O–Si) network at the monolayer’s bottom as illustrated in Fig. 1a. Its thickness was estimated to be 0.2–0.3 nm by ellipsometry. Hence, the origin of the topographic height in Fig. 2c is ascribable to the remained siloxane monolayer. Although a slight potential contrast exists between SAM1 and SAM2
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as shown in Fig. 2d, its potential contrast is only a few millivolts. This is probably due to the siloxane monolayer. Thus, we conclude that SAM1 and SAM2 are almost identical and the effect of the remained siloxane monolayer on the potential is very small. Fig. 3a–d shows KFM images of the FAS17/ODS, FAS3/ODS, AHAPS/ODS and CMPS/ODS coplanar microstructures, respectively. Rectangular features of 5 mm 25 mm correspond to the regions covered with a SAM other than ODS–SAM, while the surrounding area is covered with ODS–SAM. As clearly seen in Fig. 3a–d, the regions covered with FAS17-SAM, FAS3-SAM and CMPS-SAM seem to show lower surface potentials than ODS–SAM. On the contrary, the region covered with AHAPS-SAM, as shown in
Fig. 3. Potential images of: (a) FAS17/ODS; (b) FAS3/ODS; (c) AHAPS/ODS; (d) CMPS/ODS.
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Fig. 3c, possesses a higher surface potential than ODS–SAM. The potential contrasts of the regions covered with FAS17-SAM, FAS3-SAM, AHAPSSAM and CMPS-SAM with reference to ODS–SAM are ca. 180, 150, þ50 and 30 mV, respectively. The main advantage of ODS–SAM as a reference is its hydrophobicity. Since the amount of adsorbed water, which affects measured surface potentials significantly [20,21], on ODS–SAM is small, surface potentials are reliably measured. Here we discuss surface potential contrasts between ODS–SAM and the others. A surface potential of a SAM on a Si substrate is expressed by Eq. (1) VSAM ¼
ðfsubst ftip Þ m þ þa AeSAM e0 e
(1)
where fsubst and ftip are work functions of the Si substrate and the KFM tip, respectively, e the electric charge, m the net dipole moment directed normally to the substrate surface, A the area occupied by each molecule, and eSAM and e0 are the permittivity of the SAM and free space, respectively. Eq. (1) consists of three terms: the first is ðfsubst ftip Þ=e which
represents the contact potential difference between the Si substrate and KFM tip, the second is m/AeSAMe0, which represents the dipole moment of an organic thin film derived from Helmholtz equation, the third is a which corresponds to a potential generated by trapped charges in the SiO2 layer. The surface potential difference between the regions covered with ODS–SAM and another SAM is obtained by Eq. (2) VSAM VODS ¼
mSAM mODS ASAM eSAM e0 AODS eODS e0
(2)
where the first and third terms of Eq. (1) do not remain. We assumed that differences in A and e between ODS– SAM and the other SAMs are not so significant, since the SAMs are laterally connected with an identical Si–O–Si network as shown in Fig. 1 and are formed mainly from hydrocarbons. Thus, the potential contrast is considered to be primarily governed by the difference in dipole moment. Dipole moments of ODS, FAS17, FAS3, AHAPS and CMPS molecules were calculated and shown in Fig. 4. We employed a slightly simplified model for each molecule in which each of three methoxy groups
Fig. 4. Molecular structures and dipole moments of the model molecules for: (a) ODS; (b) FAS17; (c) FAS3; (d) AHAPS; (e) CMPS.
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is replaced with a hydrogen atom, since the methoxy groups in the precursor molecules do not remain in the SAMs. The modeled ODS, FAS17, FAS3, AHAPS and CMPS molecule have molecular lengths of 2.35, 1.34, 0.43, 1.43 and 0.62 nm and dipole moments of 2.35, 3.41, 2.91, 1.04 and 2.40 Debye, respectively. These dipole moments incline 26.5, 25.3, 3.6, 12.4 and 55.18, respectively, with respect to each of the molecular chains as shown in Fig. 4. Vertical components of the dipole moments for the model ODS and AHAPS molecules are positive, namely, the dipole moments are directing from the bottom to the top, while those for the other model molecules, i.e., FAS17, FAS3 and CMPS are negative due to electron negativities of fluorine and chlorine atoms existing in their head groups. The dipole moments of the model FAS17, FAS3, AHAPS and CMPS molecules were compared with that of the ODS model with regard to their vertical components. The estimated vertical dipole moment differences (Dm?) are as follows: Dm?ðFAS17ODSÞ ð3:92DÞ < Dm?ðFAS3ODSÞ ð3:71DÞ
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that ASAM and eSAM are identical for all the SAMs. One plausible reason for the discrepancy between the experimental and calculated results is the molecular orientation in SAM, since the above discussion is based on the assumption that molecules in a SAM are aligned perpendicular to the substrate surface. It is known that SAMs are frequently constructed from inclined molecules [1,22]. The difference in ASAM is also responsible. For example, FAS3-SAM incompletely covered a substrate compared with FAS17-SAM [20]. Since the molecular length of FAS3 is much shorter than that of FAS17, intermolecular interactions, which are necessary to form a densely packed monolayer, is so weak that FAS3-SAM is loosely packed. Further experimental results and theoretical considerations are certainly necessary in order to completely explain the surface potential contrasts between the SAMs, however, it can be concluded that the surface potential of an organosilane SAM is primarily governed by a dipole moment of the organosilane molecules consisting of the SAM.
< Dm?ðCMPSODSÞ ð1:85DÞ < Dm?ðODSODSÞ ð0DÞ < Dm?ðAHAPSODSÞ ðþ0:53DÞ
(3)
The order and signs of Dm? qualitatively agree with the order and signs of the surface potential contrasts (DV) as shown in Fig. 5. The line indicated in Fig. 5 is the relation derived form Eq. (2) with assuming
Fig. 5. Relation between Dm? and DV.
4. Conclusion Organosilane SAMs formed from ODS, FAS17, FAS3, AHAPS and CMPS have been studied by KFM and MO calculation. Surface potentials of these SAMs were measured with employing ODS–SAM as a reference. On each of the samples, this reference SAM and another SAM were microfabricated. Through this approach in which potentials of two SAMs are simultaneously measured and compared, potential contrasts between the SAM and the reference can be measured more accurately without unreproducibility which sometimes appears when measurements are conducted separately. These potential contrasts almost agreed with those predicted from the dipole moments of the corresponding precursor molecules estimated by MO calculation. Although the true dipole moment of a SAM is probably different from that of the precursor molecule due to interactions between the molecules, it was confirmed that the surface potential is primarily governed by the dipole moment of the precursor molecule. KFM will be a powerful tool for the study on structures of organic molecular monolayers, since surface potentials of the monolayers depend both on their chemical structures and packing densities.
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Acknowledgements This research has been supported by Grant-in-Aid for Scientific Research on Priority Area of ‘‘NanoMechanics of Atoms and Molecules’’ from the Ministry of Education, Culture, Sports, Science and Technology.
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