Photo and Scanning Probe Lithography Using Alkylsilane Self-Assembled Monolayers
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Mat. Res. Soc. Symp. Proc. Vol. 584 ©2000 Materials Research Society
organic contamination and to prepare a thin surface oxide layer, whose thickness was ca. 2 nm, on the substrates. This oxide surface was hydroxylated and, therefore, completely hydrophilic with a water contact angle of almost 00. The cleaned Si substrates were then exposed to OTMS vapor at
150 *C for 3 h. Through the chemical reaction between the surface hydroxyl groups and the vaporized OTMS molecules, each of the OTMS molecules was fixed onto the substrate through a Si-OSi bonding resulting in the formation of a SAM consisting of octadecylsilyl [CH 3(CH 2)17Si-, ODS] groups (ODS-SAM). The thickness of this monolayer was estimated by ellipsometry to be 1.9 ± 0.1 nm. The monolayer surface was hydrophobic showing a water contact angle of more than 100%. VUV PHOTOPATTERNING
a) I I
A
VUV light at 172 nm I I
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I I
a
I I I.
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.
I I I.
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c) I I
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Fig. 1 VUV-patteming of ODS-SAM. a) VUV-irradiation through a photomask contacting to the SAM. b) Chemical strucutres of the photoittadiated and unirradiated regions. c) An LFM image of the photopatterned ODS-SAM. d) A topographic ICAFM image of the photopatterned ODS-SAM.
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A Si substrate coated with the ODS-SAM was photolithographically micropatterned as schematically illustrated in Figs. Ia and lb. The SAM surface was irradiated for 600 s in vacuum (less than 10 Pa) with the VUV light through a photomask contacting the substrate surface. An Xe 2 2 excimer lamp at a radiation wavelength of 172 nm (Ushio Electric, UER20-172V, 10 mW/cm ) was served as the light source. The photomask consisted of a 2-mm thick quartz glass plate with a 0.1 jim-thick chromium pattern. A weight, a quartz glass plate of 10 mm thick, was put on the photomask in order to attain a satisfactory contact between the mask and the SAM surface. The total light intensity at the SAM surface was 6.5 mW/cm 2 . The photopatterned SAM surfaces were observed in the lateral force microscopy (LFM) mode using an AFM (Park Scientific Instruments, Autoprobe LS). Topographic images were also acquired in the intermittent-contact (IC) mode in which the AFM probe oscillates at a frequency near its resonance and periodically touches the sample surface once each vibration cycle. Figure Ic shows an LFM image of the ODS-SAM surface irradiated through the photomask. The bright and dark regions in the image correspond to the photoirradiated and the masked areas, respectively. Microstructures composed of 2 jim x 10 jim rectangular features were photoprinted on the SAM surfaces and were clearly imaged through the difference in friction between the photoirradiated and the unirradiated areas. The photodegraded areas, which exhibited stronger lateral force than the hydrophobic, undegraded SAM surface, had to be hydrophilic as illustrated in Fig. lb
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