- Email: [email protected]
Near-field photo-patterning of ultra-thin polymer films
Thin Solid Films 449 (2004) 226–230
Near-field photo-patterning of ultra-thin polymer films Hiroyuki Aoki*, Shinzaburo Ito Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Nishigyo, Kyoto 615-8510, Japan Received 3 June 2003; received in revised form 22 September 2003; accepted 21 October 2003
Abstract Photo-patterning of pyrene-labeled ultra-thin polymer films was investigated by scanning near-field optical microscopy and the well-defined Langmuir–Blodgett films. Pyrene was bleached by ultra-violet near-field illumination, and the recorded mark was read out as a dark region in contrast with the bright area due to fluorescence from unbleached areas. The line width for the patterned structure was much smaller than the wavelength of the excitation light. The high spatial resolution was not dependent on the scanning rate but was lowered with the increase in film thickness. For a high-resolution patterning, the recording medium is required to be thinner than the near-field region (f100 nm). The best performance for the spatial resolution, signal intensity and contrast could be obtained with a film thickness of 10–20 nm. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Langmuir–Blodgett film; Photo-patterning; Resolution
1. Introduction Optical techniques with a spatial resolution of 100 nm or less have been extensively investigated for high density memory and micro-fabrication. However, the spatial resolution of optical techniques is limited to the diffraction limit of light at a half of the wavelength (;200 nm in the visible range). An effective approach to overcome the theoretical limitation of the optical method is to lower the wavelength of the probe light. Therefore, deep UV laser and electron beam lithography techniques have been widely studied to achieve a resolution on the order of 10 nm. However, since the light at a short wavelength range and electron beam have to be used under the vacuum, it is quite difficult to reach a spatial resolution less than 100 nm in an ambient condition using the conventional optics. Recently, nearfield optics has been extensively developed to overcome the barrier of the wavelength w1–3x. Scanning near-field optical microscopy (SNOM) is one of the raster-scanned probe microscopic techniques, which illuminates the specimen by the optical near-field from a sharpened and metal-coated optical fiber probe having an aperture *Corresponding author. Tel.: q81-75-383-2613; fax: q81-75-3832617. E-mail address: [email protected] (H. Aoki).
much smaller than the wavelength of light. The nearfield ‘light’ generated at the probe end is confined in the vicinity of the aperture and does not propagate to the far-field. By approaching the probe to the sample surface one can illuminate the pointed area as small as the aperture size. Because the near-field light is able to induce changes in chemical andyor physical properties of materials, SNOM has been applied to high density optical data storage and micro-fabrication w4–9x. The high-resolution recordingyreading requires a high quality SNOM probe. Therefore, many researchers have studied the probe structure and the writingyreading out conditions to achieve the high resolution and efficiency. In the current study, we performed high resolution optical patterning on ultra-thin polymer films using SNOM and found that the spot size written by the near-field is dependent not only on the quality of the probe but also on the structure of the photosensitive layer. We discuss the optimum conditions for the near-field photo-patterning in terms of the spatial resolution and the contrast on the basis of real images obtained by the fluorescence SNOM. 2. Experiments Poly(isobutyl methacrylate) (PiBMA) was used as a base polymer of photosensitive film. The pyrene-labeled
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0040-6090Ž03.01412-3
H. Aoki, S. Ito / Thin Solid Films 449 (2004) 226–230 Table 1 Characterization of polymers
PiBMA PiBMA-Py
227
3. Results and discussion Mny103
MwyMn
296 300
1.14 1.16
PiBMA (PiBMA-Py) and unlabeled PiBMA were prepared by atom transfer radical polymerization w10,11x. Table 1 summarizes the number-average molecular weight (Mn) and the molecular weight dispersion (Mw y Mn) determined by size exclusion chromatography calibrated with polystyrene standards. The molar fraction of the pyrene unit of the labeled PiBMA was 3.0%. The sample films were prepared by the spin-coating and Langmuir–Blodgett methods to obtain the films varying in thickness. As to the thinnest sample, a monolayer of PiBMA-Py was prepared by the Langmuir–Blodgett technique. A benzene solution of PiBMA-Py at a concentration of 0.1 g ly1 was spread on a surface of pure water at 20 8C. The monolayer was compressed to a surface pressure of 5 mN my1 at a rate of 10 mm miny1. The monolayer was transferred onto a clean cover glass by the vertical dipping method at a deposition velocity of 15 mm miny1. The thickness of the PiBMA monolayer was 1.1 nm w12x. Unlabeled PiBMA doped with 1-pyrenylmethyl pivalate was spin-coated onto a cover glass from a toluene solution. The concentration of 1-pyrenylmethyl pivalate was approximately 30 mM in the resulting film, which corresponds to the pyrene concentration equivalent to the LB film. The sample films were 24, 58, 150 and 415 nm in thickness, which was determined by the height difference between the surface of the PiBMA film and the glass substrate exposed by scratching the film with a sharp needle. The details on the SNOM system used were as described elsewhere w13,14x. A He–Cd laser emitting 325- and 442-nm lines (IK5351R-D, Kimmon Electric) was used as a light source. The SNOM probe was made from an optical fiber with a pure silica core w3x, which had high transmittance in the ultra-violet region. To estimate the aperture size of the probe, fluorescent polystyrene nano-particles with a diameter of 100 nm were observed as a reference sample. In the writing process, the laser beam at a power of several milliwatts was coupled in the cleaved end of the fiber probe. The writing was carried out at various scan speed, which was defined as the linear velocity of the vector-scanning probe. The probe tip was vector-scanned along a desired trace with the near-field irradiation. The recorded pattern was read by the subsequent fluorescence SNOM imaging. The excitation wavelength was 325 nm, and the pyrene fluorescence at 360–400 nm was collected.
The scanning of a SNOM probe and the shutter control of the incident laser light allowed to draw ‘line art’ on a photosensitive polymer film. A 325-nm laser beam at a power of 4 mW was coupled in the cleaved end of the fiber probe for writing, and the power was reduced to 1 mW for the readout process. Fig. 1 shows the pyrene fluorescence SNOM image of Japanese Kanji characters meaning near-field written on a PiBMA-Py monolayer. The trace of the scanned probe was darkly measured in the fluorescence image due to the bleaching of pyrene molecules. The line width was 170 nm, which was defined as the full width at half maximum (fwhm) for the intensity profile across the line in the SNOM image. The width of the line in the read image corresponds to the convolution of the point-spread functions of the probe tip for the writing and reading processes, which were determined by the near-field distribution around the aperture w8x. In this experiment, since the writing and reading were sequentially performed using the same probe, it is valid to assume that the tip functions for both processes are the same. Assuming that the point-spread function was Gaussian, the typical width for the written line was estimated to be 120 nm, which was obviously beyond the diffraction limit of light. In the near-field recording by the aperture-type SNOM probe, the effect of heat at the tip end should be considered carefully. Since the throughput of the SNOM probe is less than 10y6, a great part of the excitation light is absorbed by the metal coating at the subwavelength aperture, resulting in temperature elevation at the SNOM probe end w15x. Hence, the pyrene molecules might be thermally bleached with the heated SNOM tip. In order to examine the heat effect on the bleaching, we carried out the writing at resonant and off-resonant wavelengths for pyrene, which were 325 and 442 nm, respectively. Fig. 2 shows the pyrene fluorescence SNOM image for the monolayer of PiBMA-Py. The two rectangular areas denoted as A and B were irradiated with the optical near-field at wave-
Fig. 1. Japanese Kanji letters, meaning Near-Field, patterned on a pyrene-labeled PiBMA monolayer. The illumination wavelength was 325 nm. The aperture size of the probe used was 90 nm.
H. Aoki, S. Ito / Thin Solid Films 449 (2004) 226–230
228
Fig. 2. Fluorescence SNOM image of the rectangular areas irradiated with optical near-field at 325 (A) and 442 nm (B) for a pyrene-labeled PiBMA monolayer. The laser power coupled into the fiber probe was 3–4 mW for each wavelength. The irradiation and the imaging were carried out sequentially with the same probe.
lengths of 325 and 442 nm, respectively. The area A illuminated at 325 nm was darkly measured as previously shown in Fig. 1. On the other hand, the fluorescence intensity from the area B illuminated at 442 nm was at the same level as that from the surrounding unirradiated region. The power of the laser beam for writing was 3–4 mW at each wavelength. Since the light absorption by the coating metal was thought to be almost independent of the wavelength from the reflection coefficient data w16x, the heat generated at the tip end was similar for the wavelengths of 325 and 442 nm. Therefore, the areas A and B were irradiated in a similar condition except for the wavelength. Fig. 2 clearly shows that pyrene in the monolayer was not bleached with the irradiation at 442 nm where pyrene had no absorption band. This indicates that the decrease of the pyrene fluorescence was not due to the heat of the probe tip or the mechanical scratching of the monolayer. Thus, the bleaching of pyrene was induced photochemically by the electronic excitation with the near-field light at 325 nm, the reaction mechanism of which was estimated to be photo-oxidation with molecular oxygen in an atmosphere w17x. We discuss the effect of the writing speed on the line width and the recording contrast. The line width was defined as the fwhm for the Gaussian curve fitted to the obtained fluorescence intensity profile in the first readout image. The recording contrast, r, was defined as rs
I0yI I0
under the SNOM aperture because of a longer settling time, i.e. a large exposure dose, at each pixel with the near-field illumination. This writing intensity dependence of the contrast was in qualitative agreement with the result of the numerical calculation w18x. Thus, the contrast increased with lowering of the writing speed. On the other hand, the line width was independent of the scan speed. Since the near-field distribution around the SNOM probe is determined by the aperture shape and the polarization state of the incident beam, the area exposed to the optical near-field is limited to the small area pointed by the probe tip. Therefore, the line width was independent of the scanning speed. Next, the effect of the film thickness was examined. It should be noted that the aperture size of the SNOM probe has a great influence both on the spatial resolution and on the intensity of the optical near-field. Therefore, in discussing the thickness dependence of the line width and the contrast, the writingyreading must be carried out with the same probe for the samples with different thicknesses. Since the writing mechanism is based on the photobleaching reaction of the pyrene chromophore, the excitation energy diffusion may dim the line width. The average distance between pyrene moieties is approximately 3.5 nm, which is sufficiently larger than the ¨ Forster radius, 1.0 nm for pyrene–pyrene energy transfer w19x. Because the energy migration does not occur at this pyrene concentration w20x, the diffusion of the excitation energy has no influence on the spatial resolution. Fig. 4 shows the thickness dependence of the width and the contrast. The contrast increased with the decrease of the film thickness and was the best for the monolayer. Since the optical near-field is non-propagating light and is constrained in the vicinity of the subwavelength-sized aperture, the amplitude of the electric field decreases with the increase of the distance from the tip end. Therefore, the pyrene units labeled in the
(1)
where I and I0 are the fluorescence intensities from the irradiated and unirradiated areas, respectively. Fig. 3 shows the scan speed dependence of the line width and the contrast for the monolayer of PiBMA-Py. The imaging contrast for the recorded line increased with the decrease of the scan speed. The slower the scan speed is, the higher number of the dye molecules bleach
Fig. 3. Scan speed dependence of the line width and the contrast for PiBMA-Py monolayer. The closed and open circles indicate the width and the contrast, respectively. The aperture size of the probe used was 180 nm.
H. Aoki, S. Ito / Thin Solid Films 449 (2004) 226–230
Fig. 4. Film thickness dependence of the line width and the contrast. The closed and open circles indicate the width and the contrast, respectively. The scan speed was 0.2 mm sy1. The probe used was identical to that for Fig. 3.
monolayer were most efficiently photobleached because they were placed just on the surface under the probe tip. On the other hand, for a thick film, there exist many dye molecules below the surface. Since the electric field amplitude steeply decreases with the distance from the aperture, the pyrene molecules apart from the surface were not bleached, resulting in a low contrast due to the strong background fluorescence from the remaining pyrene units. In addition to the characteristics of the optical near-field, it should be noted that the experiments were carried out in the atmosphere. The photobleaching of pyrene is known to occur efficiently in the presence of oxygen. Because of the oxygen diffusion from the atmosphere, the photobleaching reaction was probably enhanced near the sample surface. The increase of film thickness also resulted in the broadening of the line width. The optical near-field diverges from the sub-wavelength-sized aperture with the distance of separation from the tip end w2,21x, that is, the illuminated area increases with the distance from the probe. The highest resolution was achieved for the monolayer sample since the pyrene molecules exist only in the vicinity of the SNOM probe. If the photoreaction of pyrene was induced only by the near-field illumination confined around the aperture, an increase of line width would not be observed because the distance extends the range of the optical near-field. For the samples thicker than a few 100 nm, Fig. 4 clearly shows the increase of the line width beyond the near-field region. Such thickness dependence can be explained by the effect of the scattered light of the optical near-field. At the surface a part of the optical near-field emanating from the aperture is scattered and converted to the propagating far-field light. The scattered component can illuminate the pyrene molecules apart from the film surface and the irradiated area increases with the distance from the probe end, resulting in the broadening of the line width for the thick samples.
229
Fig. 5. Thickness dependence of the SyN ratio for the recorded line. The scan speed was 0.2 mm sy1 . The probe used was identical to that for Fig. 3.
These results indicate that the use of ultra-thin films thinner than the near-field region is crucially important in order to achieve the high spatial resolution and large contrast recording by SNOM. Thus, the dye-doped polymer monolayer is the most promising candidate for the near-field optical storage medium. Although the pyrene-labeled monolayer gave the best resolution and contrast, the signal intensity was so weak that it was difficult to obtain a sufficient signal-to-noise (SyN) ratio due to the scarcity of the dye molecule. Fig. 5 shows the film thickness dependence of the SyN ratio, which was determined from the data indicated in Fig. 4. The SyN ratio increased with the film thickness, and the film thicker than 20 nm did not show appreciable improvement in SyN ratio. Therefore, the film with a thickness of 10–20 nm shows the optimum performance of the signal intensity without degradation in the spatial resolution and contrast. Besides the high quality probe tip, the use of well-defined ultra-thin films is another key to obtaining optimum recording medium, satisfying the high resolution, large contrast, and good SyN ratio. 4. Conclusion The photo-patterning on polymer ultra-thin films was performed using SNOM. The width of the line recorded on a monolayer was less than 170 nm, which was lower than the diffraction limit of light. The thickness dependence on the line width and the contrast was examined, and it was found that the ultra-thin film made by the LB technique provides a high spatial resolution and contrast. Taking into account the SyN ratio for detection, it was concluded that the well-defined ultra-thin photosensitive film with a thickness of 10–20 nm can provide the best performance as a near-field recording medium. Acknowledgments This work was supported by the Foundation Advanced Technology Institute and Grant-in-Aid (No. 12305061)
230
H. Aoki, S. Ito / Thin Solid Films 449 (2004) 226–230
from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. References w1x E. Betzig, J.K. Trautman, Science 257 (1992) 189. w2x M.A. Paesler, P.J. Moyer, Near-Field Optics: Theory, Instrumentation, and Applications, Wiley, New York, 1996. w3x M. Ohtsu (Ed.), Near-Field NanoyAtom Optics and Technology, Springer, Tokyo, 1998. w4x E. Betzig, J.K. Trautman, R. Wolfe, E.M. Gyorgy, P.L. Finn, M.H. Kryder, C.-H. Chang, Appl. Phys. Lett. 61 (1992) 142. w5x M.K. Herndon, R.T. Collins, R.E. Hollingsworth, P.R. Larson, M.B. Johnson, Appl. Phys. Lett. 74 (1999) 141. w6x M. Irie, H. Ishida, T. Tsujioka, Jap. J. Appl. Phys. 38 (1999) 6114. w7x T. Kawai, T. Konishi, K. Matsuda, M. Irie, Jap. J. Appl. Phys. 40 (2001) 5145. w8x A. Naber, T. Dziomba, U.C. Fischer, H.-J. Maas, H. Fuchs, Appl. Phys. A 70 (2000) 227. w9x S. Sun, K.S.L. Chong, G.J. Leggett, J. Am. Chem. Soc. 124 (2002) 2414.
w10x T. Grimaud, K. Matyjaszewski, Macromolecules 30 (1997) 2216. w11x B. Yu, E. Ruckenstein, J. Polym. Sci. Part A: Polym. Chem. 37 (1999) 4191. w12x S. Ito, S. Ohmori, M. Yamamoto, Macromolecules 25 (1992) 185. w13x H. Aoki, S. Tanaka, S. Ito, M. Yamamoto, Macromolecules 33 (2000) 9650. w14x H. Aoki, S. Ito, J. Phys. Chem. B 105 (2001) 4558. w15x M. Stahelin, M.A. Bopp, G. Tarrach, A.J. Meixner, I. Zschokke-Granacher, Appl. Phys. Lett. 68 (1996) 2603. w16x S. Mononobe, Ph.D. Thesis, Tokyo Institute of Technology, Japan, 1999. w17x M.E. Sigman, P.F. Schuler, M.M. Ghosh, R.T. Dabestani, Environ. Sci. Technol. 32 (1998) 3980. w18x T. Tsujioka, M. Irie, Appl. Opt. 38 (1999) 5066. w19x I.B. Berlman, Energy Transfer Parameters of Aromatic Compounds, Academic Press, New York, 1973. w20x S. Ohmori, S. Ito, M. Yamamoto, Macromolecules 23 (1990) 4047. w21x E. Betzig, A. Harootunian, A. Lewis, M. Isaacson, Appl. Opt. 25 (1986) 1890.
Near-field photo-patterning of ultra-thin polymer films Hiroyuki Aoki*, Shinzaburo Ito Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Nishigyo, Kyoto 615-8510, Japan Received 3 June 2003; received in revised form 22 September 2003; accepted 21 October 2003
Abstract Photo-patterning of pyrene-labeled ultra-thin polymer films was investigated by scanning near-field optical microscopy and the well-defined Langmuir–Blodgett films. Pyrene was bleached by ultra-violet near-field illumination, and the recorded mark was read out as a dark region in contrast with the bright area due to fluorescence from unbleached areas. The line width for the patterned structure was much smaller than the wavelength of the excitation light. The high spatial resolution was not dependent on the scanning rate but was lowered with the increase in film thickness. For a high-resolution patterning, the recording medium is required to be thinner than the near-field region (f100 nm). The best performance for the spatial resolution, signal intensity and contrast could be obtained with a film thickness of 10–20 nm. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Langmuir–Blodgett film; Photo-patterning; Resolution
1. Introduction Optical techniques with a spatial resolution of 100 nm or less have been extensively investigated for high density memory and micro-fabrication. However, the spatial resolution of optical techniques is limited to the diffraction limit of light at a half of the wavelength (;200 nm in the visible range). An effective approach to overcome the theoretical limitation of the optical method is to lower the wavelength of the probe light. Therefore, deep UV laser and electron beam lithography techniques have been widely studied to achieve a resolution on the order of 10 nm. However, since the light at a short wavelength range and electron beam have to be used under the vacuum, it is quite difficult to reach a spatial resolution less than 100 nm in an ambient condition using the conventional optics. Recently, nearfield optics has been extensively developed to overcome the barrier of the wavelength w1–3x. Scanning near-field optical microscopy (SNOM) is one of the raster-scanned probe microscopic techniques, which illuminates the specimen by the optical near-field from a sharpened and metal-coated optical fiber probe having an aperture *Corresponding author. Tel.: q81-75-383-2613; fax: q81-75-3832617. E-mail address: [email protected] (H. Aoki).
much smaller than the wavelength of light. The nearfield ‘light’ generated at the probe end is confined in the vicinity of the aperture and does not propagate to the far-field. By approaching the probe to the sample surface one can illuminate the pointed area as small as the aperture size. Because the near-field light is able to induce changes in chemical andyor physical properties of materials, SNOM has been applied to high density optical data storage and micro-fabrication w4–9x. The high-resolution recordingyreading requires a high quality SNOM probe. Therefore, many researchers have studied the probe structure and the writingyreading out conditions to achieve the high resolution and efficiency. In the current study, we performed high resolution optical patterning on ultra-thin polymer films using SNOM and found that the spot size written by the near-field is dependent not only on the quality of the probe but also on the structure of the photosensitive layer. We discuss the optimum conditions for the near-field photo-patterning in terms of the spatial resolution and the contrast on the basis of real images obtained by the fluorescence SNOM. 2. Experiments Poly(isobutyl methacrylate) (PiBMA) was used as a base polymer of photosensitive film. The pyrene-labeled
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0040-6090Ž03.01412-3
H. Aoki, S. Ito / Thin Solid Films 449 (2004) 226–230 Table 1 Characterization of polymers
PiBMA PiBMA-Py
227
3. Results and discussion Mny103
MwyMn
296 300
1.14 1.16
PiBMA (PiBMA-Py) and unlabeled PiBMA were prepared by atom transfer radical polymerization w10,11x. Table 1 summarizes the number-average molecular weight (Mn) and the molecular weight dispersion (Mw y Mn) determined by size exclusion chromatography calibrated with polystyrene standards. The molar fraction of the pyrene unit of the labeled PiBMA was 3.0%. The sample films were prepared by the spin-coating and Langmuir–Blodgett methods to obtain the films varying in thickness. As to the thinnest sample, a monolayer of PiBMA-Py was prepared by the Langmuir–Blodgett technique. A benzene solution of PiBMA-Py at a concentration of 0.1 g ly1 was spread on a surface of pure water at 20 8C. The monolayer was compressed to a surface pressure of 5 mN my1 at a rate of 10 mm miny1. The monolayer was transferred onto a clean cover glass by the vertical dipping method at a deposition velocity of 15 mm miny1. The thickness of the PiBMA monolayer was 1.1 nm w12x. Unlabeled PiBMA doped with 1-pyrenylmethyl pivalate was spin-coated onto a cover glass from a toluene solution. The concentration of 1-pyrenylmethyl pivalate was approximately 30 mM in the resulting film, which corresponds to the pyrene concentration equivalent to the LB film. The sample films were 24, 58, 150 and 415 nm in thickness, which was determined by the height difference between the surface of the PiBMA film and the glass substrate exposed by scratching the film with a sharp needle. The details on the SNOM system used were as described elsewhere w13,14x. A He–Cd laser emitting 325- and 442-nm lines (IK5351R-D, Kimmon Electric) was used as a light source. The SNOM probe was made from an optical fiber with a pure silica core w3x, which had high transmittance in the ultra-violet region. To estimate the aperture size of the probe, fluorescent polystyrene nano-particles with a diameter of 100 nm were observed as a reference sample. In the writing process, the laser beam at a power of several milliwatts was coupled in the cleaved end of the fiber probe. The writing was carried out at various scan speed, which was defined as the linear velocity of the vector-scanning probe. The probe tip was vector-scanned along a desired trace with the near-field irradiation. The recorded pattern was read by the subsequent fluorescence SNOM imaging. The excitation wavelength was 325 nm, and the pyrene fluorescence at 360–400 nm was collected.
The scanning of a SNOM probe and the shutter control of the incident laser light allowed to draw ‘line art’ on a photosensitive polymer film. A 325-nm laser beam at a power of 4 mW was coupled in the cleaved end of the fiber probe for writing, and the power was reduced to 1 mW for the readout process. Fig. 1 shows the pyrene fluorescence SNOM image of Japanese Kanji characters meaning near-field written on a PiBMA-Py monolayer. The trace of the scanned probe was darkly measured in the fluorescence image due to the bleaching of pyrene molecules. The line width was 170 nm, which was defined as the full width at half maximum (fwhm) for the intensity profile across the line in the SNOM image. The width of the line in the read image corresponds to the convolution of the point-spread functions of the probe tip for the writing and reading processes, which were determined by the near-field distribution around the aperture w8x. In this experiment, since the writing and reading were sequentially performed using the same probe, it is valid to assume that the tip functions for both processes are the same. Assuming that the point-spread function was Gaussian, the typical width for the written line was estimated to be 120 nm, which was obviously beyond the diffraction limit of light. In the near-field recording by the aperture-type SNOM probe, the effect of heat at the tip end should be considered carefully. Since the throughput of the SNOM probe is less than 10y6, a great part of the excitation light is absorbed by the metal coating at the subwavelength aperture, resulting in temperature elevation at the SNOM probe end w15x. Hence, the pyrene molecules might be thermally bleached with the heated SNOM tip. In order to examine the heat effect on the bleaching, we carried out the writing at resonant and off-resonant wavelengths for pyrene, which were 325 and 442 nm, respectively. Fig. 2 shows the pyrene fluorescence SNOM image for the monolayer of PiBMA-Py. The two rectangular areas denoted as A and B were irradiated with the optical near-field at wave-
Fig. 1. Japanese Kanji letters, meaning Near-Field, patterned on a pyrene-labeled PiBMA monolayer. The illumination wavelength was 325 nm. The aperture size of the probe used was 90 nm.
H. Aoki, S. Ito / Thin Solid Films 449 (2004) 226–230
228
Fig. 2. Fluorescence SNOM image of the rectangular areas irradiated with optical near-field at 325 (A) and 442 nm (B) for a pyrene-labeled PiBMA monolayer. The laser power coupled into the fiber probe was 3–4 mW for each wavelength. The irradiation and the imaging were carried out sequentially with the same probe.
lengths of 325 and 442 nm, respectively. The area A illuminated at 325 nm was darkly measured as previously shown in Fig. 1. On the other hand, the fluorescence intensity from the area B illuminated at 442 nm was at the same level as that from the surrounding unirradiated region. The power of the laser beam for writing was 3–4 mW at each wavelength. Since the light absorption by the coating metal was thought to be almost independent of the wavelength from the reflection coefficient data w16x, the heat generated at the tip end was similar for the wavelengths of 325 and 442 nm. Therefore, the areas A and B were irradiated in a similar condition except for the wavelength. Fig. 2 clearly shows that pyrene in the monolayer was not bleached with the irradiation at 442 nm where pyrene had no absorption band. This indicates that the decrease of the pyrene fluorescence was not due to the heat of the probe tip or the mechanical scratching of the monolayer. Thus, the bleaching of pyrene was induced photochemically by the electronic excitation with the near-field light at 325 nm, the reaction mechanism of which was estimated to be photo-oxidation with molecular oxygen in an atmosphere w17x. We discuss the effect of the writing speed on the line width and the recording contrast. The line width was defined as the fwhm for the Gaussian curve fitted to the obtained fluorescence intensity profile in the first readout image. The recording contrast, r, was defined as rs
I0yI I0
under the SNOM aperture because of a longer settling time, i.e. a large exposure dose, at each pixel with the near-field illumination. This writing intensity dependence of the contrast was in qualitative agreement with the result of the numerical calculation w18x. Thus, the contrast increased with lowering of the writing speed. On the other hand, the line width was independent of the scan speed. Since the near-field distribution around the SNOM probe is determined by the aperture shape and the polarization state of the incident beam, the area exposed to the optical near-field is limited to the small area pointed by the probe tip. Therefore, the line width was independent of the scanning speed. Next, the effect of the film thickness was examined. It should be noted that the aperture size of the SNOM probe has a great influence both on the spatial resolution and on the intensity of the optical near-field. Therefore, in discussing the thickness dependence of the line width and the contrast, the writingyreading must be carried out with the same probe for the samples with different thicknesses. Since the writing mechanism is based on the photobleaching reaction of the pyrene chromophore, the excitation energy diffusion may dim the line width. The average distance between pyrene moieties is approximately 3.5 nm, which is sufficiently larger than the ¨ Forster radius, 1.0 nm for pyrene–pyrene energy transfer w19x. Because the energy migration does not occur at this pyrene concentration w20x, the diffusion of the excitation energy has no influence on the spatial resolution. Fig. 4 shows the thickness dependence of the width and the contrast. The contrast increased with the decrease of the film thickness and was the best for the monolayer. Since the optical near-field is non-propagating light and is constrained in the vicinity of the subwavelength-sized aperture, the amplitude of the electric field decreases with the increase of the distance from the tip end. Therefore, the pyrene units labeled in the
(1)
where I and I0 are the fluorescence intensities from the irradiated and unirradiated areas, respectively. Fig. 3 shows the scan speed dependence of the line width and the contrast for the monolayer of PiBMA-Py. The imaging contrast for the recorded line increased with the decrease of the scan speed. The slower the scan speed is, the higher number of the dye molecules bleach
Fig. 3. Scan speed dependence of the line width and the contrast for PiBMA-Py monolayer. The closed and open circles indicate the width and the contrast, respectively. The aperture size of the probe used was 180 nm.
H. Aoki, S. Ito / Thin Solid Films 449 (2004) 226–230
Fig. 4. Film thickness dependence of the line width and the contrast. The closed and open circles indicate the width and the contrast, respectively. The scan speed was 0.2 mm sy1. The probe used was identical to that for Fig. 3.
monolayer were most efficiently photobleached because they were placed just on the surface under the probe tip. On the other hand, for a thick film, there exist many dye molecules below the surface. Since the electric field amplitude steeply decreases with the distance from the aperture, the pyrene molecules apart from the surface were not bleached, resulting in a low contrast due to the strong background fluorescence from the remaining pyrene units. In addition to the characteristics of the optical near-field, it should be noted that the experiments were carried out in the atmosphere. The photobleaching of pyrene is known to occur efficiently in the presence of oxygen. Because of the oxygen diffusion from the atmosphere, the photobleaching reaction was probably enhanced near the sample surface. The increase of film thickness also resulted in the broadening of the line width. The optical near-field diverges from the sub-wavelength-sized aperture with the distance of separation from the tip end w2,21x, that is, the illuminated area increases with the distance from the probe. The highest resolution was achieved for the monolayer sample since the pyrene molecules exist only in the vicinity of the SNOM probe. If the photoreaction of pyrene was induced only by the near-field illumination confined around the aperture, an increase of line width would not be observed because the distance extends the range of the optical near-field. For the samples thicker than a few 100 nm, Fig. 4 clearly shows the increase of the line width beyond the near-field region. Such thickness dependence can be explained by the effect of the scattered light of the optical near-field. At the surface a part of the optical near-field emanating from the aperture is scattered and converted to the propagating far-field light. The scattered component can illuminate the pyrene molecules apart from the film surface and the irradiated area increases with the distance from the probe end, resulting in the broadening of the line width for the thick samples.
229
Fig. 5. Thickness dependence of the SyN ratio for the recorded line. The scan speed was 0.2 mm sy1 . The probe used was identical to that for Fig. 3.
These results indicate that the use of ultra-thin films thinner than the near-field region is crucially important in order to achieve the high spatial resolution and large contrast recording by SNOM. Thus, the dye-doped polymer monolayer is the most promising candidate for the near-field optical storage medium. Although the pyrene-labeled monolayer gave the best resolution and contrast, the signal intensity was so weak that it was difficult to obtain a sufficient signal-to-noise (SyN) ratio due to the scarcity of the dye molecule. Fig. 5 shows the film thickness dependence of the SyN ratio, which was determined from the data indicated in Fig. 4. The SyN ratio increased with the film thickness, and the film thicker than 20 nm did not show appreciable improvement in SyN ratio. Therefore, the film with a thickness of 10–20 nm shows the optimum performance of the signal intensity without degradation in the spatial resolution and contrast. Besides the high quality probe tip, the use of well-defined ultra-thin films is another key to obtaining optimum recording medium, satisfying the high resolution, large contrast, and good SyN ratio. 4. Conclusion The photo-patterning on polymer ultra-thin films was performed using SNOM. The width of the line recorded on a monolayer was less than 170 nm, which was lower than the diffraction limit of light. The thickness dependence on the line width and the contrast was examined, and it was found that the ultra-thin film made by the LB technique provides a high spatial resolution and contrast. Taking into account the SyN ratio for detection, it was concluded that the well-defined ultra-thin photosensitive film with a thickness of 10–20 nm can provide the best performance as a near-field recording medium. Acknowledgments This work was supported by the Foundation Advanced Technology Institute and Grant-in-Aid (No. 12305061)
230
H. Aoki, S. Ito / Thin Solid Films 449 (2004) 226–230
from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. References w1x E. Betzig, J.K. Trautman, Science 257 (1992) 189. w2x M.A. Paesler, P.J. Moyer, Near-Field Optics: Theory, Instrumentation, and Applications, Wiley, New York, 1996. w3x M. Ohtsu (Ed.), Near-Field NanoyAtom Optics and Technology, Springer, Tokyo, 1998. w4x E. Betzig, J.K. Trautman, R. Wolfe, E.M. Gyorgy, P.L. Finn, M.H. Kryder, C.-H. Chang, Appl. Phys. Lett. 61 (1992) 142. w5x M.K. Herndon, R.T. Collins, R.E. Hollingsworth, P.R. Larson, M.B. Johnson, Appl. Phys. Lett. 74 (1999) 141. w6x M. Irie, H. Ishida, T. Tsujioka, Jap. J. Appl. Phys. 38 (1999) 6114. w7x T. Kawai, T. Konishi, K. Matsuda, M. Irie, Jap. J. Appl. Phys. 40 (2001) 5145. w8x A. Naber, T. Dziomba, U.C. Fischer, H.-J. Maas, H. Fuchs, Appl. Phys. A 70 (2000) 227. w9x S. Sun, K.S.L. Chong, G.J. Leggett, J. Am. Chem. Soc. 124 (2002) 2414.
w10x T. Grimaud, K. Matyjaszewski, Macromolecules 30 (1997) 2216. w11x B. Yu, E. Ruckenstein, J. Polym. Sci. Part A: Polym. Chem. 37 (1999) 4191. w12x S. Ito, S. Ohmori, M. Yamamoto, Macromolecules 25 (1992) 185. w13x H. Aoki, S. Tanaka, S. Ito, M. Yamamoto, Macromolecules 33 (2000) 9650. w14x H. Aoki, S. Ito, J. Phys. Chem. B 105 (2001) 4558. w15x M. Stahelin, M.A. Bopp, G. Tarrach, A.J. Meixner, I. Zschokke-Granacher, Appl. Phys. Lett. 68 (1996) 2603. w16x S. Mononobe, Ph.D. Thesis, Tokyo Institute of Technology, Japan, 1999. w17x M.E. Sigman, P.F. Schuler, M.M. Ghosh, R.T. Dabestani, Environ. Sci. Technol. 32 (1998) 3980. w18x T. Tsujioka, M. Irie, Appl. Opt. 38 (1999) 5066. w19x I.B. Berlman, Energy Transfer Parameters of Aromatic Compounds, Academic Press, New York, 1973. w20x S. Ohmori, S. Ito, M. Yamamoto, Macromolecules 23 (1990) 4047. w21x E. Betzig, A. Harootunian, A. Lewis, M. Isaacson, Appl. Opt. 25 (1986) 1890.