- Email: [email protected]
Near-field optical microscopy in the infrared range
Micron. Vol. 27, No. 5, pp. 335 339, 1996
~ )
Copyright ~i 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved (}968 4328/96 $15.00+0.00
Pergamon PII: S0968-4328(96)00026-1
Near-field Optical Microscopy in the Infrared Range A. PIEDNOIR and F. CREUZET Laboratoirc CNRS/Saint-Gobain, "Surface du Verre et Interfaces", B.P. 135, 39, Quai Lucien Le/hanc, F-93303 Aubervilliers, France (Received 27 November 1995; revised 12 March 1996)
Abstract--A short review of sub-wavelength microscopy in the mid-infrared range is given. The home built near-field optical microscope is described, which permits the aquisition of (i) an image with a sub-wavelength spatial resolution and (ii) an infrared spectrum of an area smaller than the one investigated by conventional means, Also, we present new results which unambiguously support the specifications of the instrument. The spatial resolution is illustrated by the image of a small chessboard formed by silica patterns deposited on silicon. The locally resolved infrared spectroscopy is achieved on a 5 gm 2 sol-gel area with a spectral resolution of the order of a few wavenumbers. Finally, we shortly discuss future prospects based on the present state of the art in the visible range. Copyright ,~t", 1996 Elsevier Science Ltd
Key words: Infrared, near field, microscopy, spectroscopy, surface.
INTRODUCTION For more than one decade, near-field microscopy (Scanning Tunnelling Microscopy (STM) (Binnig et al., 1982), Atomic Force Microscopy (AFM) (Binnig et al., 1986) and Near-Field Optical Microscopy (NFOM) (Fischer, 1985; Betzig et al., 1987; Fischer and Pohl, 1989; Courjon et al., 1989; de Fornel et aL, 1989; Reddick et al., 1989; Cerre et al., 1992; Sharp et al., 1993) has brought exciting and unequalled information in surface science. For example, STM and AFM permit the imaging of crystalline structures with atomic resolution. On the other hand, NFOM has the strong potential to provide chemical information in spite of a currently poor lateral resolution (to date, the resolution in the visible range is approximately ~./60 (Zenhausern et al., 1994). For instance, fluorescence and Raman spectroscopy have been achieved (Betzig and Trautman, 1993); however, these approaches suffer from severe practical limitations which are well known and explain the large development of commercial infrared (IR) spectrometers. Consequently, NFOM working in the mid-IR range is very attractive in the sense that an ideal instrument could provide a locally resolved chemical characterization of surfaces and interfaces. This is the main motivation of our work which, as far as we know, is rather unique since (i) only attempts to realize imaging in the near (de Fornel et al., 1993) and far (Massey et al., 1985) infrared domains have been carried out and (ii) as far as spectroscopy is concerned, the work of Nakano and Kawata is somewhat limited by the use of either a CO 2 laser (Nakano and Kawata, 1993) or a prism (Nakano and Kawata, 1994). In this paper, we present a brief summary of the set-up that is used to achieve imaging with a sub-wavelength resolution in the IR range; this is illustrated by new results obtained with a silica pattern. Also, we describe the preparation of samples which appear to be very
promising (stable in time and having a good IR absorption) for the design of model samples; first spectra in the Si-H absorption band have been recorded over an area smaller than 5 ~tm2. All these results confirm that a locally resolved IR spectroscopy can be achieved after having identified the location of interest with the same instrument.
EXPERIMENTAL SET-UP Among the different configurations of NFOM, the Photon Scanning Tunnelling Microscope (PSTM) (Courjon et al., 1989; de Fornel et al., 1989; Reddick et al., 1989) is the most appealing if an extension in the IR range is to be done. As a matter of fact, two important features make it suitable: (i) the very simple requirements for optics and (ii) the relative weakness of the background signal due to the incident light. The near field is the evanescent field created by total internal reflection at the substrate-air interface; this field is locally modulated by sub-wavelength details in the vicinity of the surface. It is detected by the tip of a tapered glass fibre which can be scanned over the surface by a piezo-electric actuator or a stepper motor. Thus, an image is constructed, the contrast of which depends on both the topography and the sample absorption. Roughly, the size of the fibre tip controls the lateral resolution of the image; however, the apparent resolution might be worse if the guided signal is weak due to either an inadequate fibre shape or an insufficient light power per area unit. The feasibility of a NFOM working in the infrared range has been demonstrated in previous papers (Piednoir et al., 1995a,b). Some improvements are included in this paper. Figure 1 sketches the experimental set-up corresponding to the Photon Scanning Tunnelling Microscope working in the InfraRed (PSTM-IR). 335
336
A. Piednoir and F, Creuzet
Control, image acquisition and data processing
Boxcar integrator Reference InSb detector
X, Y and Z voltages I DC motors÷ fiezo-actuators [
hlSb'~....eetor [
Tapered fluoride
Silicon wafer
CLIO IR beam Fig. 1. Schematic set-up of the PSTM-IR.
In order to obtain a good lateral resolution, the light power per unit area must be as high as possible, and, from this point of view, the free electron laser CLIO (Collaboration pour un Laser Infrarouge ~ Orsay, LURE, France) is the best choice. The beam of this laser is collimated and quasi-monochromatic and its wavelength can be adjusted between 2 and 18 ~tm (with a spectral definition A2/2~2×10 -3) (Ortega, 1994). However, this is a pulsed source (the pulse width is about 10 ~ts with a repetition time of 20 ms) and the drawback is that the beam can not be focused since the very high power may unfortunately damage the sample. Roughly, the average power per area unit is of the order of 10 -s W/~tm2. The laser beam is totally reflected inside the silicon wafer substrate when the angle of incidence ®i is larger than the critical angle ®c. Consequently, one side of the substrate is cut at an angle of 20 ° (®i=20 °) and optically polished. This material has been chosen, not only because it is transparent in the IR range, but also because it is possible to grow a thin layer of different oxides by use of RF sputtering or sol-gel techniques. That way, it is possible to investigate samples which have a particular interest for the research activity of our laboratory. The key part of the PSTM is the detection of the evanescent field. A tip of a fluoride optical fibre (transparent in the IR up to 2=6 ~tm, to be compared with a cut-off of 2=2.2 ~tm for silica glass fibres) locally diffracts the near field and converts it into a propagative one which is guided in the fibre to the InSb detector (the band width of which is 0.6-6 ~tm). A boxcar integrator is used to average and amplify the near field signal (I) and a reference signal (Io) which permits to take into account the amplitude fluctuations of the laser beam. The ratio I/I o will be called the transmittance. When the fibre is placed above the sample surface, the-far end of the tip detects the tiny signal corresponding to the near field; unfortunately, the edges collect a large background due
to thermal effects and light diffusion. To obtain a good sensitivity (and therefore a good lateral resolution for the PSTM-IR), the background must be kept as small as possible in order to extract the signal which contains the sub-wavelength information. So, the fibre tip must be coated by a thin silver layer in order to prevent any penetration of the light by the edges of the tip. In the case of fluoride glass, it is difficult to directly evaporate the metal and chemical deposition is preferred; a layer of about 50 nm thickness is usually obtained. An image of the surface sample can be constructed by the recording of the signal during the displacement of the probe in the near field. Different means allow the fibre displacement. Micrometric screw and a piezoelectric actuator provide the coarse and fine approach, respectively, of the fibre above the sample. DC motors are preferred for in-plane displacements and 2-D scanning.
MODEL SAMPLES The very first experiments (Piednoir et al., 1995b) have shown the feasibility of PSTM-IR. The subwavelength lateral resolution has been obtained with simple objects, namely silica steps and bands deposited on a silicon wafer. SiO2 thin films (100 nm thick) were grown by ultraviolet chemical vapour deposition from a silane/oxygen mixture in a flowing nitrogen gas (Licoppe et al., 1990). For spectroscopy, the aim was to acquire an image based on a spectroscopic contrast. So, the simplest sample had to be topographically smooth but spectroscopically inhomogeneous. This is the reason why the first spectrum recorded with our PSTM-IR has been the one of a photosensitive resin. The characteristic doublet [between 2080 and 2220cm -~ (4.81 ~tm and 4.45 ~tm, respectively] of the resin disappears after an ultraviolet light exposure and it was expected to design a well-defined pattern with no topographic modifications. This material appeared to be inconvenient for adjusting the instrument because the operation must be done in the dark. For the particular work described here, two new samples have been studied. On one hand, we have chosen a more complicated pattern in order to well describe the lateral resolution of our instrument: a little silica chessboard deposited on a silicon wafer. The size of the chessboard was 100 ~tm x 100 ~tm; the side of each square is about 3.5 ~tm (smaller than the wavelength used), leading to a 2-D periodicity of 7 ~tm. The thickness of the silica layer was 100 nm. On the other hand, a new spectroscopic test-sample is a set polystyrene latex spheres (diameter 0.6 ~tm) immersed in a sol-gel layer made from triethoxysilane in acetic solution. This solution has been deposited on a silicon wafer. The spectrum of the sol-gel exhibits an absorption at 2240 cm(corresponding to a wavelength of 4.46 ~tm) characteristic of the stretching vibration Vsi_i+ This vibration has a relatively high absorption coefficient and is not present in the latex.
Near-field Optical Microscopy in the Infrared Range
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Ixm Fig. 3. Profile (continuous line in Fig. 2) unambiguously showing that the squares of the chessboard are resolved in one step (1 ~tm).
0.60 0.55 0.50 0.45 0.40 Fig. 2. 25 ~ra x 50 ~tm (25 × 50 pixels) P S T M - I R image of the silica chessboard. The CLIO wavelength is 5 p.m.
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0.35 0.30
RESULTS
0.25 0.20
For imaging, the P S T M - I R is operated in the constant altitude mode (the distance between the tip and the average plane of the substrate is kept constant while scanning the surface). In order to correct for any bad alignment between the sample and the scanning plane, the error is calculated from the position of the four corners of the image, and subsequently fed into the piezo-electric actuator.
Imaging of the silica chessboard We have assumed that the index of refraction of the silica layer is 1.5 (the same as the one of bulk silica). Therefore, the total internal reflection of the incident beam should occur at the silicon/air interface or at the silica/air interface (in this latter case, the light is refracted into the silica and the angle of incidence at the air side is about 50°). To keep the tip in the near field during the scan (the penetration depth is 1.3 ~tm for 2=5 gm, either with Oi=20 ° and n=3.41 o r O i = 5 0 ° and n=l.5), the working distance of the tip has been chosen to be 0.21am above silicon and correspondingly 0.1 ~tm above silica. Figure 2 is a PSTM-IR image (25 x 50 pixels) of the silica chessboard, recorded with a tip diameter of about 2 gm. The scanned area is 25 gm x 50 gm. The acquisition time for this image is about half an hour. We can clearly see the difference between the silicon substrate (on the top of the image) and the silica chessboard. The separation between the two domains corresponds to the perturbation of the field due to either the proximity of the pattern or to a thin silica layer which may not have been completely avoided by the masking technique. Figure 3 is a profile extracted from this image (continuous line), which clearly shows that the lateral resolution is better than the distance between two pixels (namely
4.1
4.2
4.3
4.4 4.5 4.6 4.7 4.8 wavelength(~rn) Fig. 4. Local spectrum of a thin (0.3 ~tm) sol-gel layer, as acquired with the PSTM-IR. The probed area is below the diffractionlimit.
one micron) since the boundary between silica and silicon is clearly resolved in one step. It has to be noted that the image definition depends not only on the acquisition time and the remaining noise, but also on the exact structure of the electromagnetic field near the edges of the object. In our case, the size and height of the chessboard elements are smaller than the wavelength and the light is 'diffracted' in the vicinity of each silica square• This phenomenon has been recognized on other patterns (silica step and band) with the PSTM-IR (Piednoir et al., 1995b) and other NFOMs working in the visible range (de Fornel et al., 1994). It depends both on the direction of the light beam and on the coherence of the source; theoretical investigations (de Fornel et al., 1996) are necessary to analyze the image and calculate the exact distribution of optical indexes.
Local spectroscopy After having recorded the characteristic decay of the frustrated evanescent wave, the probe is kept at a distance of about 0.15 gm from the surface; in this position, the captured intensity is mostly the near field contribution since the metallization keeps the background signal small. Subsequently, by stepping the CLIO laser wavelength through the infrared band of interest, we have acquired (in the near field regime) the local spectrum of the sol-gel solution. Figure 4 shows the measured transmittance for each wavelength around
338
A. Piednoir and F. Creuzet
Fig. 5. 30 lam × 20 ~rn (120× 20 pixels) PSTM-IR image of latex aggregates in a sol-gel solution at a CLIO wavelength of 4.70 ~tm.
ance of a latex group (height=0.6 ~tm) in the sol-gel solution (height ~0.3 ~tm). Unfortunately, no clear difference in contrast can be attributed to a spectral contribution, mainly because it is difficult to exclude the possibility of a small drift of the background and reference signals; we interpret this disappointing result as the fact that signal variation due to the topography is much more important than the one due to the absorption. The exponential decay of the evanescent field overcomes the absorption in the sol-gel. This first sample is not smooth enough to show the contrast enhancement due to absorption, and a full spectrum must be taken to obtain the chemical signature of interest.
D I S C U S S I O N AND C O N C L U S I O N
10 p.m I
I
Fig. 6.30 p.mx 20 p.m (60 x 20 pixels) PSTM IR image of latex aggregates in a sol-gel solution at a CLIO wavelength of 4.42 lam. the expected absorption peak. We can see a minimum of the transmittance for a wavelength of about 4.46 ~tm corresponding to an Si-H absorption of roughly 40 % (which is similar to the one obtained with a conventional spectrometer). The diameter of the fibre tip is about 2 ~tm and is very similar to the tip-surface distance below which the near field becomes relevant; therefore, from what is known in the visible range, we can infer that the probed area is roughly 5 ~tm2. It is important to emphasize that this locally resolved infrared spectroscopy stands close to the diffraction limit; furthermore, this spatial resolution is only limited by the sensitivity of the experiment, and possible improvements are to be expected, which will be briefly discussed in the next section. In order to investigate the capability of spectroscopic imaging, we have recorded few images at two different laser wavelengths, one at 4.70 ~tm beside the Si-H absorption peak (Fig. 5) and the other at 4.42 ~tm (Fig. 6). The size of the images is 30 p~m × 20 lam. On both images, we can clearly see a maximum of intensity (high transmission) which reveals a real feature on the left side of the surface. It is difficult to unambiguously characterize this object. However, we know from A F M measurements that, in the sol-gel solution, most of the latex spheres aggregate at the surface and groups of at least 5 spheres may form a domain larger than 0.6 ~tm; thus, a reasonable interpretation of the images is the appear-
These new results confirm that near field optical microscopy can be operated in the mid-infrared range. Locally resolved spectroscopy has been achieved on two oscillators having significantly different extinction coefficients: Si-H in a sol-gel and N = N = N + of a photosensitive resin (Piednoir et al., 1995b). To date, the spatial resolution (typically one micron) remains controlled by the sensitivity of the experiment and not by the size of the fibre tip. In order to improve this sensitivity, the major requirement is the metal-coating of the fibre edges which strongly prevents the penetration of the background light. However, it is very reasonable to question the quality of the guidance in the tip region because, (i) the shape factor is not optimal and (ii) inefficient reflexion is likely to occur at the interface between the fluoride glass and the chemically deposited silver layer. From what is known in the visible range (Buckland et al., 1993), an intensity loss by a factor of about 100 may be expected as a consequence of this bad coupling between the near field and the rest of the fibre. Future work is now under progress to avoid the fibre (since the handling of fluoride glass is difficult) and use a small silicon pyramid as the probe. In that case, a single structure (maintaining the silicon tip and the detection optics in a fixed position) will be moved by DC-motors to scan the surface with a lateral increment of about 50 nm. Another advantage of silicon is that, unlike fluoride glass, the IR bandwidth is not limited and many resonators can be studied. Finally, the use of a free electron laser might appear as a severe constraint for practical applications. It is important to say that a laboratory source like the cascade arc can be implemented with the P S T M - I R (Piednoir et al., 1995b). This source is the polychromatic and continuous light emission of a black body having an equivalent temperature of ~ 13000 K. Although the available power per unit area is one order of magnitude smaller than the one of CLIO (for a given spectral width), this light source has the potential to facilitate the operation of the instrument. The procedure for surface analysis would be to acquire the topography with the full bandwidth of the system, and acquire the spectrum
Near-field Optical Microscopy in the Infrared Range
afterwards by either the use of filters or the connection with a commercial Fourier transform spectrometer. The images obtained with this incoherent light source exhibit an oscillatory behavior (near the sharp topographic edges of the sample) which is less pronounced than the one observed with the coherent CLIO source, in agreement with the theoretical calculations (de Fornel et al., 1996). Although this work is still at an early stage, we feel that improvements in the fabrication of the probe should bring the increase of sensitivity that is required. It is worth emphasizing that this local probe microscopy is non destructive and could be applied to the study of soft materials (e.g. polymers, bio-materials). Acknowledgements
We wish to thank D. Abriou, C. Jolu and the CLIO team lk~r skilful assistance.
REFERENCES Betzig, E., Isaacson, A. and Lewis, A., 1987. Appl. Phys. Lett., 51, 2088. Betzig, E. and Trautman, J. K., 1993. Science, 257, 189. Binnig, G., Rohrer, H., Gerber, C. and Weibel, E., 1982. Appl. Phys. Lett., 40, 178, Phys. Rev. Lett., 49, 57 and Physiea, 1091110b, 2075. Binnig, G., Quate, C, F. and Gerber, C., 1986. Phys. Rev. Lett., 56, 930.
339
Buckland, E., Moyer, P. and Paesler, M., 1993. J., Appl. Phys., 73, 1018. Cerre, N., de Fornel, F. and Goudonnet, J. P., 1992. Appl. Opt., 31, 903. Courjon, D., Sarrayeddine, K. and Spajer, M., 1989. Opt. Com., 71, 23. de Fornel, F., Goudonnet, J. P., Salomon, L. and Leniewska, E., 1989. In Proc SPIE Conf., Paris, April, 1139, 77. de Fornel, F., Leniewska, E., Salomon, L. and Goudonnet, J. P., 1993. Opt. Com., 102, 1. de Fornel, F., Adam, P., Salomon, L. and Goudonnet, J. P., 1994. Opt. Lett., 19, 1082. deFornel, F., Adam, P. M., Salomon, L., Goudonnet, J. P., Sentenac, A., Carminati, R. and Greffet, J. J., 1996. Opt Soc Am. A., 13, 35 36. Fischer, U. Ch., 1985. ,L Vac. TechnoL. B 3, 386. Fischer, U. Ch. and Pohl, D. W., 1989. Phys. Rev. Lett.. 62, 458. Licoppe, C., Wattine, F., Meriadec, C., Flicstein, J. and Nissim, Y. I., 1990. Z Appl. Phys., 68, 5636. Massey, G. A., Davis, J. A., Katnick, S. M. and Omon, E., 1985. Appl. Opt. 24, 1498. Nakano, T. and Kawata, S., 1993. Optik, 94, 159. Nakano, T. and Kawata, S., 1994. Scanning, 16, 368. Ortega, J. M., t994. Nucl. Instr. and Meth. A, 341, 138. Piednoir, A., Creuzet, F., Licoppe, C. and Ortega, J. M., 1995a. Ultramicroscopy, 57, 282. Piednoir, A., Licoppe, C. and Creuzet, F., 1995b. Opt. Com., submitted. Reddick, R. C., Warmack, R. J. and Ferrell, T. L., 1989. Phys. Rev. B, 39, 767. Sharp, S. L., Warmack, R. J., Goudonnet, J. P., Lee, I. and FerreU, T. L., 1993. Acc. Chem. Research, 26, 377. Zenhausern, F., O'Boyle, M. P. and Wickramasinghe, H. K., 1994. Appl. Phys. Lett., 65, 1623.
~ )
Copyright ~i 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved (}968 4328/96 $15.00+0.00
Pergamon PII: S0968-4328(96)00026-1
Near-field Optical Microscopy in the Infrared Range A. PIEDNOIR and F. CREUZET Laboratoirc CNRS/Saint-Gobain, "Surface du Verre et Interfaces", B.P. 135, 39, Quai Lucien Le/hanc, F-93303 Aubervilliers, France (Received 27 November 1995; revised 12 March 1996)
Abstract--A short review of sub-wavelength microscopy in the mid-infrared range is given. The home built near-field optical microscope is described, which permits the aquisition of (i) an image with a sub-wavelength spatial resolution and (ii) an infrared spectrum of an area smaller than the one investigated by conventional means, Also, we present new results which unambiguously support the specifications of the instrument. The spatial resolution is illustrated by the image of a small chessboard formed by silica patterns deposited on silicon. The locally resolved infrared spectroscopy is achieved on a 5 gm 2 sol-gel area with a spectral resolution of the order of a few wavenumbers. Finally, we shortly discuss future prospects based on the present state of the art in the visible range. Copyright ,~t", 1996 Elsevier Science Ltd
Key words: Infrared, near field, microscopy, spectroscopy, surface.
INTRODUCTION For more than one decade, near-field microscopy (Scanning Tunnelling Microscopy (STM) (Binnig et al., 1982), Atomic Force Microscopy (AFM) (Binnig et al., 1986) and Near-Field Optical Microscopy (NFOM) (Fischer, 1985; Betzig et al., 1987; Fischer and Pohl, 1989; Courjon et al., 1989; de Fornel et aL, 1989; Reddick et al., 1989; Cerre et al., 1992; Sharp et al., 1993) has brought exciting and unequalled information in surface science. For example, STM and AFM permit the imaging of crystalline structures with atomic resolution. On the other hand, NFOM has the strong potential to provide chemical information in spite of a currently poor lateral resolution (to date, the resolution in the visible range is approximately ~./60 (Zenhausern et al., 1994). For instance, fluorescence and Raman spectroscopy have been achieved (Betzig and Trautman, 1993); however, these approaches suffer from severe practical limitations which are well known and explain the large development of commercial infrared (IR) spectrometers. Consequently, NFOM working in the mid-IR range is very attractive in the sense that an ideal instrument could provide a locally resolved chemical characterization of surfaces and interfaces. This is the main motivation of our work which, as far as we know, is rather unique since (i) only attempts to realize imaging in the near (de Fornel et al., 1993) and far (Massey et al., 1985) infrared domains have been carried out and (ii) as far as spectroscopy is concerned, the work of Nakano and Kawata is somewhat limited by the use of either a CO 2 laser (Nakano and Kawata, 1993) or a prism (Nakano and Kawata, 1994). In this paper, we present a brief summary of the set-up that is used to achieve imaging with a sub-wavelength resolution in the IR range; this is illustrated by new results obtained with a silica pattern. Also, we describe the preparation of samples which appear to be very
promising (stable in time and having a good IR absorption) for the design of model samples; first spectra in the Si-H absorption band have been recorded over an area smaller than 5 ~tm2. All these results confirm that a locally resolved IR spectroscopy can be achieved after having identified the location of interest with the same instrument.
EXPERIMENTAL SET-UP Among the different configurations of NFOM, the Photon Scanning Tunnelling Microscope (PSTM) (Courjon et al., 1989; de Fornel et al., 1989; Reddick et al., 1989) is the most appealing if an extension in the IR range is to be done. As a matter of fact, two important features make it suitable: (i) the very simple requirements for optics and (ii) the relative weakness of the background signal due to the incident light. The near field is the evanescent field created by total internal reflection at the substrate-air interface; this field is locally modulated by sub-wavelength details in the vicinity of the surface. It is detected by the tip of a tapered glass fibre which can be scanned over the surface by a piezo-electric actuator or a stepper motor. Thus, an image is constructed, the contrast of which depends on both the topography and the sample absorption. Roughly, the size of the fibre tip controls the lateral resolution of the image; however, the apparent resolution might be worse if the guided signal is weak due to either an inadequate fibre shape or an insufficient light power per area unit. The feasibility of a NFOM working in the infrared range has been demonstrated in previous papers (Piednoir et al., 1995a,b). Some improvements are included in this paper. Figure 1 sketches the experimental set-up corresponding to the Photon Scanning Tunnelling Microscope working in the InfraRed (PSTM-IR). 335
336
A. Piednoir and F, Creuzet
Control, image acquisition and data processing
Boxcar integrator Reference InSb detector
X, Y and Z voltages I DC motors÷ fiezo-actuators [
hlSb'~....eetor [
Tapered fluoride
Silicon wafer
CLIO IR beam Fig. 1. Schematic set-up of the PSTM-IR.
In order to obtain a good lateral resolution, the light power per unit area must be as high as possible, and, from this point of view, the free electron laser CLIO (Collaboration pour un Laser Infrarouge ~ Orsay, LURE, France) is the best choice. The beam of this laser is collimated and quasi-monochromatic and its wavelength can be adjusted between 2 and 18 ~tm (with a spectral definition A2/2~2×10 -3) (Ortega, 1994). However, this is a pulsed source (the pulse width is about 10 ~ts with a repetition time of 20 ms) and the drawback is that the beam can not be focused since the very high power may unfortunately damage the sample. Roughly, the average power per area unit is of the order of 10 -s W/~tm2. The laser beam is totally reflected inside the silicon wafer substrate when the angle of incidence ®i is larger than the critical angle ®c. Consequently, one side of the substrate is cut at an angle of 20 ° (®i=20 °) and optically polished. This material has been chosen, not only because it is transparent in the IR range, but also because it is possible to grow a thin layer of different oxides by use of RF sputtering or sol-gel techniques. That way, it is possible to investigate samples which have a particular interest for the research activity of our laboratory. The key part of the PSTM is the detection of the evanescent field. A tip of a fluoride optical fibre (transparent in the IR up to 2=6 ~tm, to be compared with a cut-off of 2=2.2 ~tm for silica glass fibres) locally diffracts the near field and converts it into a propagative one which is guided in the fibre to the InSb detector (the band width of which is 0.6-6 ~tm). A boxcar integrator is used to average and amplify the near field signal (I) and a reference signal (Io) which permits to take into account the amplitude fluctuations of the laser beam. The ratio I/I o will be called the transmittance. When the fibre is placed above the sample surface, the-far end of the tip detects the tiny signal corresponding to the near field; unfortunately, the edges collect a large background due
to thermal effects and light diffusion. To obtain a good sensitivity (and therefore a good lateral resolution for the PSTM-IR), the background must be kept as small as possible in order to extract the signal which contains the sub-wavelength information. So, the fibre tip must be coated by a thin silver layer in order to prevent any penetration of the light by the edges of the tip. In the case of fluoride glass, it is difficult to directly evaporate the metal and chemical deposition is preferred; a layer of about 50 nm thickness is usually obtained. An image of the surface sample can be constructed by the recording of the signal during the displacement of the probe in the near field. Different means allow the fibre displacement. Micrometric screw and a piezoelectric actuator provide the coarse and fine approach, respectively, of the fibre above the sample. DC motors are preferred for in-plane displacements and 2-D scanning.
MODEL SAMPLES The very first experiments (Piednoir et al., 1995b) have shown the feasibility of PSTM-IR. The subwavelength lateral resolution has been obtained with simple objects, namely silica steps and bands deposited on a silicon wafer. SiO2 thin films (100 nm thick) were grown by ultraviolet chemical vapour deposition from a silane/oxygen mixture in a flowing nitrogen gas (Licoppe et al., 1990). For spectroscopy, the aim was to acquire an image based on a spectroscopic contrast. So, the simplest sample had to be topographically smooth but spectroscopically inhomogeneous. This is the reason why the first spectrum recorded with our PSTM-IR has been the one of a photosensitive resin. The characteristic doublet [between 2080 and 2220cm -~ (4.81 ~tm and 4.45 ~tm, respectively] of the resin disappears after an ultraviolet light exposure and it was expected to design a well-defined pattern with no topographic modifications. This material appeared to be inconvenient for adjusting the instrument because the operation must be done in the dark. For the particular work described here, two new samples have been studied. On one hand, we have chosen a more complicated pattern in order to well describe the lateral resolution of our instrument: a little silica chessboard deposited on a silicon wafer. The size of the chessboard was 100 ~tm x 100 ~tm; the side of each square is about 3.5 ~tm (smaller than the wavelength used), leading to a 2-D periodicity of 7 ~tm. The thickness of the silica layer was 100 nm. On the other hand, a new spectroscopic test-sample is a set polystyrene latex spheres (diameter 0.6 ~tm) immersed in a sol-gel layer made from triethoxysilane in acetic solution. This solution has been deposited on a silicon wafer. The spectrum of the sol-gel exhibits an absorption at 2240 cm(corresponding to a wavelength of 4.46 ~tm) characteristic of the stretching vibration Vsi_i+ This vibration has a relatively high absorption coefficient and is not present in the latex.
Near-field Optical Microscopy in the Infrared Range
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Ixm Fig. 3. Profile (continuous line in Fig. 2) unambiguously showing that the squares of the chessboard are resolved in one step (1 ~tm).
0.60 0.55 0.50 0.45 0.40 Fig. 2. 25 ~ra x 50 ~tm (25 × 50 pixels) P S T M - I R image of the silica chessboard. The CLIO wavelength is 5 p.m.
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0.35 0.30
RESULTS
0.25 0.20
For imaging, the P S T M - I R is operated in the constant altitude mode (the distance between the tip and the average plane of the substrate is kept constant while scanning the surface). In order to correct for any bad alignment between the sample and the scanning plane, the error is calculated from the position of the four corners of the image, and subsequently fed into the piezo-electric actuator.
Imaging of the silica chessboard We have assumed that the index of refraction of the silica layer is 1.5 (the same as the one of bulk silica). Therefore, the total internal reflection of the incident beam should occur at the silicon/air interface or at the silica/air interface (in this latter case, the light is refracted into the silica and the angle of incidence at the air side is about 50°). To keep the tip in the near field during the scan (the penetration depth is 1.3 ~tm for 2=5 gm, either with Oi=20 ° and n=3.41 o r O i = 5 0 ° and n=l.5), the working distance of the tip has been chosen to be 0.21am above silicon and correspondingly 0.1 ~tm above silica. Figure 2 is a PSTM-IR image (25 x 50 pixels) of the silica chessboard, recorded with a tip diameter of about 2 gm. The scanned area is 25 gm x 50 gm. The acquisition time for this image is about half an hour. We can clearly see the difference between the silicon substrate (on the top of the image) and the silica chessboard. The separation between the two domains corresponds to the perturbation of the field due to either the proximity of the pattern or to a thin silica layer which may not have been completely avoided by the masking technique. Figure 3 is a profile extracted from this image (continuous line), which clearly shows that the lateral resolution is better than the distance between two pixels (namely
4.1
4.2
4.3
4.4 4.5 4.6 4.7 4.8 wavelength(~rn) Fig. 4. Local spectrum of a thin (0.3 ~tm) sol-gel layer, as acquired with the PSTM-IR. The probed area is below the diffractionlimit.
one micron) since the boundary between silica and silicon is clearly resolved in one step. It has to be noted that the image definition depends not only on the acquisition time and the remaining noise, but also on the exact structure of the electromagnetic field near the edges of the object. In our case, the size and height of the chessboard elements are smaller than the wavelength and the light is 'diffracted' in the vicinity of each silica square• This phenomenon has been recognized on other patterns (silica step and band) with the PSTM-IR (Piednoir et al., 1995b) and other NFOMs working in the visible range (de Fornel et al., 1994). It depends both on the direction of the light beam and on the coherence of the source; theoretical investigations (de Fornel et al., 1996) are necessary to analyze the image and calculate the exact distribution of optical indexes.
Local spectroscopy After having recorded the characteristic decay of the frustrated evanescent wave, the probe is kept at a distance of about 0.15 gm from the surface; in this position, the captured intensity is mostly the near field contribution since the metallization keeps the background signal small. Subsequently, by stepping the CLIO laser wavelength through the infrared band of interest, we have acquired (in the near field regime) the local spectrum of the sol-gel solution. Figure 4 shows the measured transmittance for each wavelength around
338
A. Piednoir and F. Creuzet
Fig. 5. 30 lam × 20 ~rn (120× 20 pixels) PSTM-IR image of latex aggregates in a sol-gel solution at a CLIO wavelength of 4.70 ~tm.
ance of a latex group (height=0.6 ~tm) in the sol-gel solution (height ~0.3 ~tm). Unfortunately, no clear difference in contrast can be attributed to a spectral contribution, mainly because it is difficult to exclude the possibility of a small drift of the background and reference signals; we interpret this disappointing result as the fact that signal variation due to the topography is much more important than the one due to the absorption. The exponential decay of the evanescent field overcomes the absorption in the sol-gel. This first sample is not smooth enough to show the contrast enhancement due to absorption, and a full spectrum must be taken to obtain the chemical signature of interest.
D I S C U S S I O N AND C O N C L U S I O N
10 p.m I
I
Fig. 6.30 p.mx 20 p.m (60 x 20 pixels) PSTM IR image of latex aggregates in a sol-gel solution at a CLIO wavelength of 4.42 lam. the expected absorption peak. We can see a minimum of the transmittance for a wavelength of about 4.46 ~tm corresponding to an Si-H absorption of roughly 40 % (which is similar to the one obtained with a conventional spectrometer). The diameter of the fibre tip is about 2 ~tm and is very similar to the tip-surface distance below which the near field becomes relevant; therefore, from what is known in the visible range, we can infer that the probed area is roughly 5 ~tm2. It is important to emphasize that this locally resolved infrared spectroscopy stands close to the diffraction limit; furthermore, this spatial resolution is only limited by the sensitivity of the experiment, and possible improvements are to be expected, which will be briefly discussed in the next section. In order to investigate the capability of spectroscopic imaging, we have recorded few images at two different laser wavelengths, one at 4.70 ~tm beside the Si-H absorption peak (Fig. 5) and the other at 4.42 ~tm (Fig. 6). The size of the images is 30 p~m × 20 lam. On both images, we can clearly see a maximum of intensity (high transmission) which reveals a real feature on the left side of the surface. It is difficult to unambiguously characterize this object. However, we know from A F M measurements that, in the sol-gel solution, most of the latex spheres aggregate at the surface and groups of at least 5 spheres may form a domain larger than 0.6 ~tm; thus, a reasonable interpretation of the images is the appear-
These new results confirm that near field optical microscopy can be operated in the mid-infrared range. Locally resolved spectroscopy has been achieved on two oscillators having significantly different extinction coefficients: Si-H in a sol-gel and N = N = N + of a photosensitive resin (Piednoir et al., 1995b). To date, the spatial resolution (typically one micron) remains controlled by the sensitivity of the experiment and not by the size of the fibre tip. In order to improve this sensitivity, the major requirement is the metal-coating of the fibre edges which strongly prevents the penetration of the background light. However, it is very reasonable to question the quality of the guidance in the tip region because, (i) the shape factor is not optimal and (ii) inefficient reflexion is likely to occur at the interface between the fluoride glass and the chemically deposited silver layer. From what is known in the visible range (Buckland et al., 1993), an intensity loss by a factor of about 100 may be expected as a consequence of this bad coupling between the near field and the rest of the fibre. Future work is now under progress to avoid the fibre (since the handling of fluoride glass is difficult) and use a small silicon pyramid as the probe. In that case, a single structure (maintaining the silicon tip and the detection optics in a fixed position) will be moved by DC-motors to scan the surface with a lateral increment of about 50 nm. Another advantage of silicon is that, unlike fluoride glass, the IR bandwidth is not limited and many resonators can be studied. Finally, the use of a free electron laser might appear as a severe constraint for practical applications. It is important to say that a laboratory source like the cascade arc can be implemented with the P S T M - I R (Piednoir et al., 1995b). This source is the polychromatic and continuous light emission of a black body having an equivalent temperature of ~ 13000 K. Although the available power per unit area is one order of magnitude smaller than the one of CLIO (for a given spectral width), this light source has the potential to facilitate the operation of the instrument. The procedure for surface analysis would be to acquire the topography with the full bandwidth of the system, and acquire the spectrum
Near-field Optical Microscopy in the Infrared Range
afterwards by either the use of filters or the connection with a commercial Fourier transform spectrometer. The images obtained with this incoherent light source exhibit an oscillatory behavior (near the sharp topographic edges of the sample) which is less pronounced than the one observed with the coherent CLIO source, in agreement with the theoretical calculations (de Fornel et al., 1996). Although this work is still at an early stage, we feel that improvements in the fabrication of the probe should bring the increase of sensitivity that is required. It is worth emphasizing that this local probe microscopy is non destructive and could be applied to the study of soft materials (e.g. polymers, bio-materials). Acknowledgements
We wish to thank D. Abriou, C. Jolu and the CLIO team lk~r skilful assistance.
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