- Email: [email protected]
Transmission scanning near-field optical microscopy with uncoated silicon tips
Ultramicroscopy 71 (1998) 371—377
Transmission scanning near-field optical microscopy with uncoated silicon tips Hans U. Danzebrink!,*, Annick Castiaux", Christian Girard#, Xavier Bouju#, Gu¨nter Wilkening! ! Physikalisch-Technische Bundesanstalt, Bundesallee 100, D-38116 Braunschweig, Germany " Laboratoire de Physique du Solide, Facultes Universitaires Notre Dame de la Paix, B-5000 Namur, Belgium # Laboratoire de Physique Mole& culaire, Universite& de Franche Comte& , F-25030 Besanc7 on Cedex, France
Abstract In this paper we report on the implementation of an uncoated silicon (Si) cantilever probe into a transmission scanning near-field optical microscopy (SNOM) architecture. In a first stage, the expected transmission behaviour of a sharp silicon probe is investigated by calculating the complete electric field distribution both inside and outside a silicon tip facing a sample. Experimental applications using near-infrared radiation (j"1.06 lm) are then proposed. In particular, compact disc features (*x)1 lm) were imaged successfully with our setup (lateral resolution: better than 250 nm). Furthermore, when dealing with finer sample structures (*x)100 nm), topography artifacts were clearly evidenced. The resulting highly resolved images of nanostructures are to be attributed to some interference effects occurring between the illuminated probe and the sample. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 07.79.!v; 07.79.Fc; 07.79.Lh Keywords: Near-field optical microscopy (NFOM); Atomic-force microscopy (AFM); Tip-scanning instrumentation design and characterization
1. Introduction The recent developments of photon local-probebased devices have led to many different experi-
* Corresponding author. Tel.: #49 531 592 5136; fax: #49 531 592 5105; e-mail: [email protected].
mental architectures [1—3]. Among the different configurations, the transmission mode is the oldest technique used in NFO [4,5]. In addition to providing high-resolution NFO images, one of the principal advantages of the transmission SNOM is its ability to deal with surface corrugations of large amplitude [6], in contrast to other SNOM configurations based on a TIR-PSTM setup, where the problem of scattering is dominant.
0304-3991/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 0 4 - 3 9 9 1 ( 9 7 ) 0 0 1 0 1 - 0
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H.U. Danzebrink et al. / Ultramicroscopy 71 (1998) 371—377
In the past, two different setups have been developed from this basic configuration: (i) The first widely used technique is the transmission illumination mode in which the light radiated by a nanometric emitter is converted into propagating waves by the sample itself. Actually, a nanometric aperture is not necessary if the effective interaction area with the sample is small enough in relation to the incident wavelength. (ii) The second configuration, the so-called transmission collection mode, is easily obtained by reversing the light path. In this case, the aperture plays the role of a nano-collector, and the light converted in the near-field zone is transmitted to a photodetector. In this paper, we discuss some preliminary results recorded with an experimental setup based on the implementation of an Si cantilever into a transmission illumination mode-SNOM architecture. In the recent years, several groups have developed similar devices using Si N pyramidal tips for si3 4 multaneous SNOM and AFM (e.g. Refs. [7—10]). Nevertheless, until now only a few attempts have been made to build up microscopes that integrate Si tips [11—17].
2. Advantages of using silicon probes The general advantages of AFM cantilever probes (Si N or Si) are obvious. These probes are 3 4 microfabricated in batch processes, and they are easy to use in an AFM configuration with the risk being rather low that they are crashed like the fiber probes which are comparatively rigid in the direction towards the sample. Compared with Si N 3 4 which is also used as a probe material, Si has some unique properties. First of all, the high refractive index of silicon in the optical range (n"3.9 at j"633 nm) as well as for infrared wavelengths (n"3.5 for j"1.1—15 lm) considerably shortens the effective wavelength inside the probe material. This property increases the transmission efficiency through the tip, for emission as well as for detection, compared with the tips commonly used which have a lower refractive index (n"1.45 in the visible range). However, for the sake of completeness, a disadvantage of silicon
should be mentioned: Its refractive index has a non-vanishing imaginary part for wavelengths in the visible, which is of the order of 0.02. This leads to light absorption inside the material, which may be neglected if the distances involved are small ((2 lm), but which can cause real problems if the electromagnetic wave has to travel a long way inside a silicon fiber, for instance. In this respect, glass continues to be a material advantageous for use in the visible range. However, looking at higher wavelengths, in the infrared spectrum, the imaginary part of the refractive index vanishes, and no more absorption is to be expected. Moreover, the optical parameters remain constant throughout the infrared spectrum, which makes silicon an ideal probe material to realize infrared spectroscopy (silicon is transparent even up to j"15 lm). Some more practical considerations prove that this material is particularly interesting. A very important technical advantage results from the possibility of fabricating stiff cantilevers for the noncontact AFM mode (C"10—100 N/m compared with C"0.02—0.5 N/m of contact mode cantilevers). These stiff cantilever probes can be used not only for simultaneous AFM (non-contact mode) and SNOM, but also for constant height scans at very small tip-surface distances without “jump to contact” (see Fig. 5C). Finally, silicon cantilevers can be used as passive [11—13] or active (optoelectronic) near-field probes [14—17], depending on the optical wavelength of the incident electromagnetic field (light transmission or absorption in the Si). In contrast to the passive probes where the tip is used as a waveguide or scattering center, the active probes allow the photodetection process of the optical signal to be integrated into the near-field probe itself. The electric measuring signal can then be directly picked up at the probe’s electrodes, and no external photodetectors are required.
3. Computerized simulations of the transmission behaviour of silicon tips In transmission illumination mode-SNOM the use of a pointed probe allows the radiative optical field to be converted into confined optical fields
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concentrated near the probe’s front tip. The amount of optical energy converted by such devices depends strongly on both the shape and the optical properties of the probe [18]. In this section, in order to demonstrate the exceptional transmission behaviour of silicon tips used in emission, we report in Fig. 1 numerical results obtained with a twodimensional model of an Si probe facing an Si surface. This simulation is based on the localized Green’s function approach [19,20] in which the reference system is a silicon medium. We chose to model an acute tip, with an aperture angle of 30°. The tip is completely dielectric, except for a small metal screen at its bottom that is assumed to center the incident beam to the axis of the tip. The incident electromagnetic field is a Gaussian wave (FWHM"4 lm) coming from a semi-infinite silicon medium. Its wavelength is 1.064 lm, i.e. a near-infrared wavelength where the Si material is non-absorbing. In front of the tip, at a distance of z"0 nm (Fig. 1A: for s-polarization) a semi-infinite silicon medium was taken as the sample. In the second case (Fig. 1B) the distance from the surface is 75 nm. For this polarization (p-polarization) the signal dependence from the tip-surface distance is not significant, so the electric field distribution would not change for smaller distances. The grey-scale maps of Fig. 1A and Fig. 1B present the distribution of the total electric field ampli-
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tude, for s- and p-polarizations, respectively. In s-polarization the electric field vector oscillates perpendicular to the image plane. As mentioned before, the large index of refraction of silicon reduces the effective wavelength inside the tip, and the spolarized wave is easily guided down to the apex of the tip, even with the acute shape. Fig. 1A even shows a maximum of the field near the front tip. The behaviour of the p-polarized wave is completely different. This polarization gives rise to depolarization effects on the boundary surfaces of the tip, but also on the surface facing the tip. This is due to the boundary conditions imposed by Maxwell’s equations on the fields perpendicular to an interface. Discontinuities appear at these interfaces. Propagation inside the tip seems more difficult for this polarization so that only little light reaches the apex of the tip. For this polarization, a central decay surrounded by higher intensities appear as a delocalized background illumination. The two-dimensional calculations make possible a qualitative understanding of the behaviour of real (three-dimensional) Si tips. In the experiments, the three-dimensional tips support both polarizations simultaneously, and the effects explained before are combined. Silicon tips produce a well-confined and intense central spot, from the s-polarization, sometimes with a small central decay due to depolarization effects occurring in the p-polarization. The
Fig. 1. Distribution of the total electric field amplitude inside an uncoated Si probe facing an Si sample (two-dimensional model). The figures show the electric field resulting from an incident Gaussian wave coming from the right side, for s-(A) and p-polarization (B), respectively. In s-polarization the electric field vector oscillates perpendicular to the image plane.
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Fig. 2. Three cuts through different field calculations. All cuts are taken along the x-axis just below the surface of the Si sample. The results for the s-polarization (z"0 nm) and for the ppolarization (z"75 nm) are taken from Fig. 1A and Fig. 1B, respectively. For the final resolution of the microscope, of course, the spot size of the light intensity is relevant. The intensity pattern would have a width being even smaller than 200 nm.
Fig. 3. Transmission SNOM setup (the detection scheme for the AFM distance control is not shown). The sample is scanned, while the cantilever and laser remain in a fixed position.
well-confined spot (size)j/4) should be dominant if the tip-surface distance is small enough for an efficient coupling of the evanescent field from the probe to the sample. In Fig. 2 three cuts through different field calculations are shown. All cuts are taken along the x-axis just below the surface of the Si sample. The width of the emission pattern for s-polarization is in the order of 225 nm (FWHM) in contact with the sample (280 nm at z"75 nm).
4. Results and discussion We adopted the illumination mode (setup in Fig. 3) and constructed a force microscope with the option of transmission illumination for the SNOM. In order to get the highest possible NA for the detection, the sample was directly glued onto the photodiode using immersion oil. All SNOM images were made using unmetallized Si tips. We imaged compact disc (CD) structures. The CD sample was cut by diamond turning so that some holes were still filled with metal and others were empty (Fig. 4) [21]. In the near-field measurements (Fig. 5) we noticed that empty holes (dark in
Fig. 4. Survey of the CD sample (photograph of the image taken by a conventional transmission microscope, size: about 64 lm]47 lm); the sketch below shows a cut through the sample: Section A: empty pits: due to the turning process the plastic is drawn out of the pits together with the metal (cf. Fig. 5A: holes in the AFM image); Section B: only some of the metal fillings are removed; Section C: no metal is removed, so this area is dark in the transmission image.
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Fig. 5. Simultaneous AFM (contact mode) (A) and SNOM (B) imaging of the CD structures. SNOM at constant height (z+100 nm) for the same area (C); (photo signal amplitude: +180—200 mV, depth of the structures free from metal: +135 nm, height of the metallized structures: +40 nm).
the AFM image) have the same grey level as the background in the SNOM image. Holes filled with metal are dark because light is absorbed by the metal. The SNOM image (Fig. 5C) taken by keeping the height at a constant level without any distance control (z+100 nm) shows similar results. In this case, fine structures which are visible in Fig. 5B disappeared due to the increased spot size of the diverging beam (cf. Fig. 1A). The lateral resolution in the constant height images is better than 250 nm (edge resolution, 20—80% criterion). This corresponds very well to the spot size determined from the simulation results. In constant height images there are no topography-related “optical” signals due, for example, to bending of the cantilever or changing of the contact area between tip and surfacffle edges, and topography-induced interference effects could be excluded (see below).
Thus, it has been shown that structures of the size of the wavelength can be imaged. To investigate sub-wavelength resolution, a fine structure sample (“Fischer-sample” [22,23]) was used. Highresolution “SNOM images” were obtained (Fig. 6). In this case it turned out, however, that constant height scanning did not show any contrast in the “SNOM image”. The contrast in Fig. 6B is, therefore, a topography artifact which arises from the variation of the distance between probe and surface occurring when the tip follows the surface profile. The resulting signal mainly originates from interference effects appearing between probe and sample. The fraction of the signal due to absorption by the small metal triangles is negligible as the triangles are small compared with the area which is illuminated by the tip (see Fig. 1A and Fig. 1B). The transmission behaviour of the probe-sample
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Fig. 6. Simultaneous AFM (A) and SNOM (B) imaging of the latex spheres samples (see Ref. [23]); (photo signal amplitude: +35—40 mV, height of the triangle structures: +20 nm).
Fig. 7. Approach curve determined on the sample of Fig. 6 (optical and force signals versus distance). The zero value of the “mV” scale is arbitrary. The horizontal axis indicates the “z” scale of the cantilever movement (approach—retraction). The contact point is determined by the onset of the deflection of the cantilever (see force signal, showing the force—distance curve of the AFM).
interferometer can be seen in Fig. 7. The optical signal oscillates when the distance between probe and surface is varied. From the z-values of the measurement represented in Fig. 6, a 40 mV change of the light intensity of a 20 nm step can be deduced. From Fig. 7 follows a 400 mV change of intensity for a tip—surface distance variation of *z"200 nm. Both results fit well and confirm the assumption that the pattern shown in Fig. 6B is dominated by topography-related interference effects.
In the case of Fig. 5 the situation is different. Here, the influence of interference effects appears to be small because the areas with strong topographical changes (unmetallized holes: *z+135 nm) generate small optical signals (+35 mV), whereas nearly flat metallized structures (*z+40 nm) induce large optical signals (+200 mV). The structure size of this sample is larger than the effectively illuminated area, so these structures could be resolved by the uncoated Si tips. Hence, the optical contrast is mainly due to index (absorption) changes. Topography effects do not dominate in this case.
5. Conclusion It has been shown that transmission SNOM with uncoated Si tips works in the case of structures ()j) where the influence of the absorption is greater than that of the interference effects. A lateral resolution of approximately 250 nm was achieved in constant height scans. For small structures ((100 nm), the absorption becomes negligible compared with the interference effects, due to the fact that the effectively illuminated area is large compared with the size of the structures. Therefore, it should be pointed out that although we call our microscope a near-field microscope the main field components involved in the imaging process using uncoated tips are far-field components [24]. Of
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course, aperture probes would be a solution to overcome these restrictions. However, there are still some other possibilities of improving the image quality and the resolution of uncoated Si probes. First, the background photodetector signal should be reduced by imaging only the bright spot from the front tip (confocal arrangement). Also, a shorter wavelength leads to further confinement of the light spot. In that case the absorption of the silicon has to be taken into account and a probe with a reduced tip height must be used. Finally, a change from the transmission setup to a microscope working in reflection by using the tip both as an emitter and a detector (“internal reflection mode”) [1,6] would result in an improvement of the resolution.
Acknowledgements The authors would like to thank G. Hinzmann and V. Ja¨ger for preparing the diamond turned CD samples and H. Wolff for the microphotographs and the fabrication of the mechanical parts of our SNOM. Thanks are due to Dr. U. C. Fischer (University of Mu¨nster) for valuable discussions and for providing us with the “Fischer-samples”. Dr. O. Ohlsson (Nanosensors Dr. Olaf Wolter GmbH) supplied us with Si cantilever probes. This work was financially supported by the European Community (HCM-contract No. CHRXCT930130) and by the Volkswagen Stiftung.
References [1] D. Courjon, C. Bainier, Rep. Prog. Phys. 57 (1994) 989. [2] J.P. Fillard, Near Field Optics and Nanoscopy, World Scientific, Singapore, 1996. [3] M.A. Paesler, P.J. Moyer, Near-Field Optics: Theory, Instrumentation, and Applications, Wiley-Interscience, New York, 1996.
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[4] E. Betzig, A. Harootunian, A. Lewis, M. Isaacson, Appl. Opt. 25 (1986) 1890. [5] U. Du¨rig, D.W. Pohl, F. Rohner, J. Appl. Phys. 59 (1986) 3318. [6] E. Betzig, M. Isaacson, H. Barshatzky, A. Lewis, K. Lin, SPIE vol. 897 (1988) 91. [7] N.F. van Hulst, M.H.P. Moers, O.F.J. Noordman, T. Faulkner, F.B. Segerink, K.O. van der Werf, B.G. de Grooth, B. Bo¨lger, SPIE vol. 1639 (1992) 36. [8] F. Baida, D. Courjon, G. Tribillon, in: D.W. Pohl, D. Courjon (Eds.), Near Field Optics, Kluwer, Dordrecht, 1993, p. 71. [9] M. Radmacher, P.E. Hillner, P.K. Hansma, Rev. Sci. Instrum. 65 (1994) 2737. [10] A.G.T. Ruiter, M.H.P. Moers, A. Jalocha, N.F. van Hulst, Ultramicroscopy 61 (1995) 139. [11] J.P. Fillard, M. Castagne, C. Prioleau, M. Benfedda, J. Bonnafe, Ultramicroscopy 61 (1995) 85. [12] J.P. Fillard, M. Castagne, M. Benfedda, S. Lahimer, H.-U. Danzebrink, Appl. Phys. A 63 (1996) 421. [13] F. Zenhausern, M.P. O’Boyle, H.K. Wickramasinghe, Appl. Phys. Lett. 65 (1994) 1623. [14] H.U. Danzebrink, G. Wilkening, O. Ohlsson, Appl. Phys. Lett. 67 (1995) 1981. [15] H.U. Danzebrink, O. Ohlsson, G. Wilkening, Ultramicroscopy 61 (1995) 131. [16] R.C. Davis, C.C. Williams, P. Neuzil, Appl. Phys. Lett. 66 (1995) 2309. [17] S. Akamine, H. Kuwano, H. Yamada, Appl. Phys. Lett. 68 (1996) 579. [18] Ch. Girard, A. Dereux, Rep. Prog. Phys. 59 (1996) 657. [19] A. Castiaux, A. Dereux, J.P. Vigneron, Ch. Girard, O.J.F. Martin, Ultramicroscopy 60 (1995) 1. [20] A. Castiaux, Ch. Girard, A. Dereux, O.F.J. Martin, J.P. Vigneron, Phys. Rev. E 54 (1996) 5752. [21] The original aim was to fabricate a sample with metal structures buried under a flat surface. However, due to the turning process the plastic together with the metal was drawn out of the pits and the resulting surface was not flat within some 10 nms. [22] U.C. Fischer, H.P. Zingsheim, J. vac. Sci. Technol. 19 (1981) 881. [23] A latex monolayer is deposited on glass (latex sphere diameter +450 nm, glass thickness: 150 lm), on which an Al layer 20 nm thick is evaporated. The latex spheres are then removed in an ultrasonic bath and triangular-shaped Al structures with dimensions (100 nm remain on the glass. [24] V. Sandoghdar, S. Wegscheider, G. Krausch, J. Mlynek, J. Appl. Phys. 81 (1997) 2499.
Transmission scanning near-field optical microscopy with uncoated silicon tips Hans U. Danzebrink!,*, Annick Castiaux", Christian Girard#, Xavier Bouju#, Gu¨nter Wilkening! ! Physikalisch-Technische Bundesanstalt, Bundesallee 100, D-38116 Braunschweig, Germany " Laboratoire de Physique du Solide, Facultes Universitaires Notre Dame de la Paix, B-5000 Namur, Belgium # Laboratoire de Physique Mole& culaire, Universite& de Franche Comte& , F-25030 Besanc7 on Cedex, France
Abstract In this paper we report on the implementation of an uncoated silicon (Si) cantilever probe into a transmission scanning near-field optical microscopy (SNOM) architecture. In a first stage, the expected transmission behaviour of a sharp silicon probe is investigated by calculating the complete electric field distribution both inside and outside a silicon tip facing a sample. Experimental applications using near-infrared radiation (j"1.06 lm) are then proposed. In particular, compact disc features (*x)1 lm) were imaged successfully with our setup (lateral resolution: better than 250 nm). Furthermore, when dealing with finer sample structures (*x)100 nm), topography artifacts were clearly evidenced. The resulting highly resolved images of nanostructures are to be attributed to some interference effects occurring between the illuminated probe and the sample. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 07.79.!v; 07.79.Fc; 07.79.Lh Keywords: Near-field optical microscopy (NFOM); Atomic-force microscopy (AFM); Tip-scanning instrumentation design and characterization
1. Introduction The recent developments of photon local-probebased devices have led to many different experi-
* Corresponding author. Tel.: #49 531 592 5136; fax: #49 531 592 5105; e-mail: [email protected].
mental architectures [1—3]. Among the different configurations, the transmission mode is the oldest technique used in NFO [4,5]. In addition to providing high-resolution NFO images, one of the principal advantages of the transmission SNOM is its ability to deal with surface corrugations of large amplitude [6], in contrast to other SNOM configurations based on a TIR-PSTM setup, where the problem of scattering is dominant.
0304-3991/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 0 4 - 3 9 9 1 ( 9 7 ) 0 0 1 0 1 - 0
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H.U. Danzebrink et al. / Ultramicroscopy 71 (1998) 371—377
In the past, two different setups have been developed from this basic configuration: (i) The first widely used technique is the transmission illumination mode in which the light radiated by a nanometric emitter is converted into propagating waves by the sample itself. Actually, a nanometric aperture is not necessary if the effective interaction area with the sample is small enough in relation to the incident wavelength. (ii) The second configuration, the so-called transmission collection mode, is easily obtained by reversing the light path. In this case, the aperture plays the role of a nano-collector, and the light converted in the near-field zone is transmitted to a photodetector. In this paper, we discuss some preliminary results recorded with an experimental setup based on the implementation of an Si cantilever into a transmission illumination mode-SNOM architecture. In the recent years, several groups have developed similar devices using Si N pyramidal tips for si3 4 multaneous SNOM and AFM (e.g. Refs. [7—10]). Nevertheless, until now only a few attempts have been made to build up microscopes that integrate Si tips [11—17].
2. Advantages of using silicon probes The general advantages of AFM cantilever probes (Si N or Si) are obvious. These probes are 3 4 microfabricated in batch processes, and they are easy to use in an AFM configuration with the risk being rather low that they are crashed like the fiber probes which are comparatively rigid in the direction towards the sample. Compared with Si N 3 4 which is also used as a probe material, Si has some unique properties. First of all, the high refractive index of silicon in the optical range (n"3.9 at j"633 nm) as well as for infrared wavelengths (n"3.5 for j"1.1—15 lm) considerably shortens the effective wavelength inside the probe material. This property increases the transmission efficiency through the tip, for emission as well as for detection, compared with the tips commonly used which have a lower refractive index (n"1.45 in the visible range). However, for the sake of completeness, a disadvantage of silicon
should be mentioned: Its refractive index has a non-vanishing imaginary part for wavelengths in the visible, which is of the order of 0.02. This leads to light absorption inside the material, which may be neglected if the distances involved are small ((2 lm), but which can cause real problems if the electromagnetic wave has to travel a long way inside a silicon fiber, for instance. In this respect, glass continues to be a material advantageous for use in the visible range. However, looking at higher wavelengths, in the infrared spectrum, the imaginary part of the refractive index vanishes, and no more absorption is to be expected. Moreover, the optical parameters remain constant throughout the infrared spectrum, which makes silicon an ideal probe material to realize infrared spectroscopy (silicon is transparent even up to j"15 lm). Some more practical considerations prove that this material is particularly interesting. A very important technical advantage results from the possibility of fabricating stiff cantilevers for the noncontact AFM mode (C"10—100 N/m compared with C"0.02—0.5 N/m of contact mode cantilevers). These stiff cantilever probes can be used not only for simultaneous AFM (non-contact mode) and SNOM, but also for constant height scans at very small tip-surface distances without “jump to contact” (see Fig. 5C). Finally, silicon cantilevers can be used as passive [11—13] or active (optoelectronic) near-field probes [14—17], depending on the optical wavelength of the incident electromagnetic field (light transmission or absorption in the Si). In contrast to the passive probes where the tip is used as a waveguide or scattering center, the active probes allow the photodetection process of the optical signal to be integrated into the near-field probe itself. The electric measuring signal can then be directly picked up at the probe’s electrodes, and no external photodetectors are required.
3. Computerized simulations of the transmission behaviour of silicon tips In transmission illumination mode-SNOM the use of a pointed probe allows the radiative optical field to be converted into confined optical fields
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concentrated near the probe’s front tip. The amount of optical energy converted by such devices depends strongly on both the shape and the optical properties of the probe [18]. In this section, in order to demonstrate the exceptional transmission behaviour of silicon tips used in emission, we report in Fig. 1 numerical results obtained with a twodimensional model of an Si probe facing an Si surface. This simulation is based on the localized Green’s function approach [19,20] in which the reference system is a silicon medium. We chose to model an acute tip, with an aperture angle of 30°. The tip is completely dielectric, except for a small metal screen at its bottom that is assumed to center the incident beam to the axis of the tip. The incident electromagnetic field is a Gaussian wave (FWHM"4 lm) coming from a semi-infinite silicon medium. Its wavelength is 1.064 lm, i.e. a near-infrared wavelength where the Si material is non-absorbing. In front of the tip, at a distance of z"0 nm (Fig. 1A: for s-polarization) a semi-infinite silicon medium was taken as the sample. In the second case (Fig. 1B) the distance from the surface is 75 nm. For this polarization (p-polarization) the signal dependence from the tip-surface distance is not significant, so the electric field distribution would not change for smaller distances. The grey-scale maps of Fig. 1A and Fig. 1B present the distribution of the total electric field ampli-
373
tude, for s- and p-polarizations, respectively. In s-polarization the electric field vector oscillates perpendicular to the image plane. As mentioned before, the large index of refraction of silicon reduces the effective wavelength inside the tip, and the spolarized wave is easily guided down to the apex of the tip, even with the acute shape. Fig. 1A even shows a maximum of the field near the front tip. The behaviour of the p-polarized wave is completely different. This polarization gives rise to depolarization effects on the boundary surfaces of the tip, but also on the surface facing the tip. This is due to the boundary conditions imposed by Maxwell’s equations on the fields perpendicular to an interface. Discontinuities appear at these interfaces. Propagation inside the tip seems more difficult for this polarization so that only little light reaches the apex of the tip. For this polarization, a central decay surrounded by higher intensities appear as a delocalized background illumination. The two-dimensional calculations make possible a qualitative understanding of the behaviour of real (three-dimensional) Si tips. In the experiments, the three-dimensional tips support both polarizations simultaneously, and the effects explained before are combined. Silicon tips produce a well-confined and intense central spot, from the s-polarization, sometimes with a small central decay due to depolarization effects occurring in the p-polarization. The
Fig. 1. Distribution of the total electric field amplitude inside an uncoated Si probe facing an Si sample (two-dimensional model). The figures show the electric field resulting from an incident Gaussian wave coming from the right side, for s-(A) and p-polarization (B), respectively. In s-polarization the electric field vector oscillates perpendicular to the image plane.
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Fig. 2. Three cuts through different field calculations. All cuts are taken along the x-axis just below the surface of the Si sample. The results for the s-polarization (z"0 nm) and for the ppolarization (z"75 nm) are taken from Fig. 1A and Fig. 1B, respectively. For the final resolution of the microscope, of course, the spot size of the light intensity is relevant. The intensity pattern would have a width being even smaller than 200 nm.
Fig. 3. Transmission SNOM setup (the detection scheme for the AFM distance control is not shown). The sample is scanned, while the cantilever and laser remain in a fixed position.
well-confined spot (size)j/4) should be dominant if the tip-surface distance is small enough for an efficient coupling of the evanescent field from the probe to the sample. In Fig. 2 three cuts through different field calculations are shown. All cuts are taken along the x-axis just below the surface of the Si sample. The width of the emission pattern for s-polarization is in the order of 225 nm (FWHM) in contact with the sample (280 nm at z"75 nm).
4. Results and discussion We adopted the illumination mode (setup in Fig. 3) and constructed a force microscope with the option of transmission illumination for the SNOM. In order to get the highest possible NA for the detection, the sample was directly glued onto the photodiode using immersion oil. All SNOM images were made using unmetallized Si tips. We imaged compact disc (CD) structures. The CD sample was cut by diamond turning so that some holes were still filled with metal and others were empty (Fig. 4) [21]. In the near-field measurements (Fig. 5) we noticed that empty holes (dark in
Fig. 4. Survey of the CD sample (photograph of the image taken by a conventional transmission microscope, size: about 64 lm]47 lm); the sketch below shows a cut through the sample: Section A: empty pits: due to the turning process the plastic is drawn out of the pits together with the metal (cf. Fig. 5A: holes in the AFM image); Section B: only some of the metal fillings are removed; Section C: no metal is removed, so this area is dark in the transmission image.
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Fig. 5. Simultaneous AFM (contact mode) (A) and SNOM (B) imaging of the CD structures. SNOM at constant height (z+100 nm) for the same area (C); (photo signal amplitude: +180—200 mV, depth of the structures free from metal: +135 nm, height of the metallized structures: +40 nm).
the AFM image) have the same grey level as the background in the SNOM image. Holes filled with metal are dark because light is absorbed by the metal. The SNOM image (Fig. 5C) taken by keeping the height at a constant level without any distance control (z+100 nm) shows similar results. In this case, fine structures which are visible in Fig. 5B disappeared due to the increased spot size of the diverging beam (cf. Fig. 1A). The lateral resolution in the constant height images is better than 250 nm (edge resolution, 20—80% criterion). This corresponds very well to the spot size determined from the simulation results. In constant height images there are no topography-related “optical” signals due, for example, to bending of the cantilever or changing of the contact area between tip and surfacffle edges, and topography-induced interference effects could be excluded (see below).
Thus, it has been shown that structures of the size of the wavelength can be imaged. To investigate sub-wavelength resolution, a fine structure sample (“Fischer-sample” [22,23]) was used. Highresolution “SNOM images” were obtained (Fig. 6). In this case it turned out, however, that constant height scanning did not show any contrast in the “SNOM image”. The contrast in Fig. 6B is, therefore, a topography artifact which arises from the variation of the distance between probe and surface occurring when the tip follows the surface profile. The resulting signal mainly originates from interference effects appearing between probe and sample. The fraction of the signal due to absorption by the small metal triangles is negligible as the triangles are small compared with the area which is illuminated by the tip (see Fig. 1A and Fig. 1B). The transmission behaviour of the probe-sample
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Fig. 6. Simultaneous AFM (A) and SNOM (B) imaging of the latex spheres samples (see Ref. [23]); (photo signal amplitude: +35—40 mV, height of the triangle structures: +20 nm).
Fig. 7. Approach curve determined on the sample of Fig. 6 (optical and force signals versus distance). The zero value of the “mV” scale is arbitrary. The horizontal axis indicates the “z” scale of the cantilever movement (approach—retraction). The contact point is determined by the onset of the deflection of the cantilever (see force signal, showing the force—distance curve of the AFM).
interferometer can be seen in Fig. 7. The optical signal oscillates when the distance between probe and surface is varied. From the z-values of the measurement represented in Fig. 6, a 40 mV change of the light intensity of a 20 nm step can be deduced. From Fig. 7 follows a 400 mV change of intensity for a tip—surface distance variation of *z"200 nm. Both results fit well and confirm the assumption that the pattern shown in Fig. 6B is dominated by topography-related interference effects.
In the case of Fig. 5 the situation is different. Here, the influence of interference effects appears to be small because the areas with strong topographical changes (unmetallized holes: *z+135 nm) generate small optical signals (+35 mV), whereas nearly flat metallized structures (*z+40 nm) induce large optical signals (+200 mV). The structure size of this sample is larger than the effectively illuminated area, so these structures could be resolved by the uncoated Si tips. Hence, the optical contrast is mainly due to index (absorption) changes. Topography effects do not dominate in this case.
5. Conclusion It has been shown that transmission SNOM with uncoated Si tips works in the case of structures ()j) where the influence of the absorption is greater than that of the interference effects. A lateral resolution of approximately 250 nm was achieved in constant height scans. For small structures ((100 nm), the absorption becomes negligible compared with the interference effects, due to the fact that the effectively illuminated area is large compared with the size of the structures. Therefore, it should be pointed out that although we call our microscope a near-field microscope the main field components involved in the imaging process using uncoated tips are far-field components [24]. Of
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course, aperture probes would be a solution to overcome these restrictions. However, there are still some other possibilities of improving the image quality and the resolution of uncoated Si probes. First, the background photodetector signal should be reduced by imaging only the bright spot from the front tip (confocal arrangement). Also, a shorter wavelength leads to further confinement of the light spot. In that case the absorption of the silicon has to be taken into account and a probe with a reduced tip height must be used. Finally, a change from the transmission setup to a microscope working in reflection by using the tip both as an emitter and a detector (“internal reflection mode”) [1,6] would result in an improvement of the resolution.
Acknowledgements The authors would like to thank G. Hinzmann and V. Ja¨ger for preparing the diamond turned CD samples and H. Wolff for the microphotographs and the fabrication of the mechanical parts of our SNOM. Thanks are due to Dr. U. C. Fischer (University of Mu¨nster) for valuable discussions and for providing us with the “Fischer-samples”. Dr. O. Ohlsson (Nanosensors Dr. Olaf Wolter GmbH) supplied us with Si cantilever probes. This work was financially supported by the European Community (HCM-contract No. CHRXCT930130) and by the Volkswagen Stiftung.
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