Progress in photon scanning tunneling microscopy (PSTM)

Ultramicroscopy 42-44 (1992) 41)8-415 North-Holland

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Progress in photon scanning tunneling microscopy (PSTM) T . L . F e r r e l l , S.L. S h a r p a n d R . J . W a r m a c k Oak Ridge National Laboratory *, Oak Ridge, TN 37831, USA

Received 12 August 1991

Tunneling of either electrons or photons from a sample to a probe tip yields an exponential signal that permits subwavelength tangential resolution and even better resolution of the tip-to-sample distance. PSTM provides images of both insulators and conductors that are either thin or not too strongly absorbing at the wavelength used. STM provides better spatial resolution but poorer spectroscopic resolution, so there are complementary aspects for the two instruments. Our recent progress with the PSTM includes adding a zoom capability, detection of surface-enhanced Raman signals, imaging of photolithographically prepared arrays of isolated silver particles, and studies of tunneling effects on films of photoresist.

1. Introduction E x t r a o r d i n a r y p r o g r e s s has b e e n m a d e in the past d e c a d e in both p h o t o n a n d e l e c t r o n scanning m i c r o s c o p y by taking a d v a n t a g e of the e x p o n e n tial c h a r a c t e r of tunneling. T h e p h o t o n s c a n n i n g t u n n e l i n g m i c r o s c o p e ( P S T M ) [1,2J utilizes this e x p o n e n t i a l in the s a m e way as does the e l e c t r o n STM, and the P S T M occupies a special niche a m o n g all forms of microscopy; it p r o v i d e s spatial r e s o l u t i o n at a small fraction of the w a v e l e n g t h of the e l e c t r o m a g n e t i c r a d i a t i o n used and provides high-intensity, s p e c t r o s c o p i c r e s o l u t i o n of 10 7 eV. T r a n s p a r e n t or thin s a m p l e s can be t o p o graphically i m a g e d , high-quality optical surfaces can be i n s p e c t e d a n d optical i n t e g r a t e d circuits can be quantitatively p r o b e d without c o n t a c t (the P S T M thus acting as an optical V O M ) . C h e m i c a l m a p p i n g can be c a r r i e d out using several different forms of e l e c t r o m a g n e t i c spectroscopy, microstress p a t t e r n s can be d e t e c t e d with p o l a r i z e d light, s u b m i c r o m e t e r optical l i t h o g r a p h y can be a c c o m p l i s h e d in a new way (thus taking advan* Sponsored by the Office of Health and Environmental Research, US Department of Energy, under contract DEAC05-84OR214()0 with Martin Marietta Energy Systems, Inc.

tage of p h o t o r e s i s t technology), standing-wave scattering, diffraction, a n d i n t e r f e r e n c e can be directly o b s e r v e d for a variety of objects p l a c e d on a m i c r o s c o p e slide, surface p l a s m o n s can be s t u d i e d in detail, n o n l i n e a r e l e c t r o m a g n e t i c phen o m e n a can be locally p r o b e d , and work is now b e g i n n i n g on e x t e n d i n g the P S T M into the soft X-ray regime w h e r e m a n y new a p p l i c a t i o n s await. T h e P S T M also offers an a l t e r n a t i v e way of r e a d ing D N A s e q u e n c e s t h r o u g h the local d e t e c t i o n of R a m a n signals from f e r r o c i n e - t a g g e d base pairs, and a n u m b e r of o t h e r biological applications are c u r r e n t l y being c a r r i e d out. Success in these e n d e a v o r s is p r i m a r i l y d u e to the e x p o n e n tial decay of the t u n n e l i n g signal a t t a i n e d from f r u s t r a t i n g the total internal reflection of p h o t o n s in a s a m p l e by using a s h a r p e n e d optical fiber probe. W h i l e the t r a n s m i s s i o n coefficient for tunneling o f ' s - p o l a r i z e d ' p h o t o n s is identical to that for electrons, the v e c t o r n a t u r e of the p h o t o n wave functions c o m e s into play by yielding a different t r a n s m i s s i o n coefficient for ' p - p o l a r i z e d ' photons. T h e p o l a r i z a t i o n , e n e r g y a n d angle of incidence of the p h o t o n s can be c o n t r o l l e d in the P S T M and this r e p r e s e n t s a nice a d v a n t a g e for both imaging a n d s p e c t r o s c o p i c work. Below we give several e x a m p l e s of r e c e n t p r o g r e s s m a d e in P S T M research. O u r c u r r e n t

0304-3991/92/$//5.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

T.L. Ferrell et al. / Progress in photon scanning tunneling microscopy

PSTM's have several forms, but include one that utilizes an optical microscope to gain a zoom capability and is basically the same instrument as is produced commercially [3]. The controller and other electronics are exactly the same as those used in STM work. Instrumentation is further described below.

2. Experimental details The systems we have utilized to date ordinarily provide total internal reflection of a stable, lowpower, laser b e a m in a quartz hemicylinder. A white light source may be used if various wavelengths are focused at appropriately chosen angles inside the hemicylinder to provide identical decay lengths for all colors. High-index hemicylinders, e.g., diamond, have also been used. Fig. 1 shows a system diagram and fig. 2 shows the

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PSTM image of the surface of a diamond hemicylinder provided by a commercial source [4] for testing by our Group. Usually the sample is mounted on an optically flat quartz microscope slide and the slide is placed on the hemicylinder with an index-matching gel between the two surfaces. Internal reflection is designed to occur at the sample surface. A movable piano-concave lens at the entrance face to the hemicylinder eliminates unwanted deviations while a wedge is used at the exit face to prevent reflected light which would return to the sample (except when the reflection is useful). The probe tip is an etched quartz optical fiber mounted on a cylindrical piezoelectric scanner and passing through a connector to a photomultiplier tube. Constant height or constant intensity scans can be carried out. Fig. 3 shows a side-byside comparison of the two scan modes over a region of quartz containing a polystyrene marker.

Fig. 1. Block diagram of the PSTM. A spectroscopy option can be added using a split optical fiber.

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T.L. Ferrell et al. / Progress in photon scanning tunneling microscopy

Fig. 2. Surface of a polished diamond hemicylinder as observed by the PSTM. The image represents a scan of 10× 10 ktm,

Fig. 3. Comparison of constant-height and constant-intensity scan modes of the PSTM for a polystyrene marker on a quartz slide The images are basically the same.

T.L. Ferrell et al. / Progress in photon scanning tunneling microscopy

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Fig. 4. Block diagram of the apparatus for performing serially prepared photolithography on the sub-micron scale with the PSTM. Exposure occurs due to tunneling current J and is prevented at sub-transparency threshold intensities occurring outside the region of significant J (the photo-bleachable dye is opaque below a threshold intensity); exposure is highest near the quartz slide; H e - N e laser is used for imaging and H e - C d laser for lithography.

Fig. 5. Local exposure of photoresist with the PSTM tunneling current. The total scan is 2 x 2 p,m. The dark spot in the exposure pattern corresponds to a groove observed in the tip (see fig. 6).

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T.L. Ferrell et al. / Progress in photon scanning tunneling microscopy

Fig. 6. Scanning electron microscope image of the PSTM probe tip used for the photolithography of fig. 5.

The images are essentially the same. Typically the probe tip is initially brought to the surface until the exponential decay curve is acquired and then a scan mode is selected. For photolithographic work the system is shown in fig. 4. A photobleachable dye layer is coated onto the sample slide and covered with photoresist. The dye becomes transparent above an intensity threshold attained from the tunneling flux under the probe tip. Our first result was just recently obtained and is shown in fig. 5 as a simple local exposure that reflects the tip's shape. The dimensions clearly indicate that we should be able to write masks with linewidths of 100 nm or better. Fig. 6 shows the probe used for the exposure. The small groove in the tip is demonstrated as the dark spot in the exposure shown in fig. 5. In order to acquire the exponential curve without danger of crashing the tip, and also to measure local variations in the index of refraction (thereby preventing confusion with topographical variations), it is useful to oscillate the tip. A lock-in amplifier allows us to acquire the oscillating current's amplitude as a function of the direct current. The slope of this curve yields the ratio of the spatial amplitude A to the exponential decay length b (which in turn yields the index of refraction). For small A/b the plot is linear.

The PSTM apparatus includes a spectrometer and filters when chemical identification is needed. Several forms of spectroscopy are possible, but we have recently been most interested in surface-enhanced R a m a n spectroscopy (SERS). The SERS signal acquired by the PSTM probe tip in tunneling mode is weak compared to that obtained in other spectroscopies, but is adequate. We use a band-rejection filter and a single monochromator with a diode-array detector for rapid spectrum acquisition. SERS utilizes silver microstructures to enhance the R a m a n cross-section and we have prepared re-usable, photolithographically produced crossed gratings etched in quartz and coated with 15 nm of silver. These can be imaged with the PSTM and provide excellent SERS results, as described below. We have also prepared arrays of isolated silver particles on flat quartz. Compounds examined thus far include benzoic acid and N-methyl formamide. Details are given in another p a p e r presented at this conference [5]. We have provided instrumentation and technical aid to other groups interested in PSTM spectroscopy and some of the results on fluorescence spectroscopy have been published [6].

3. Applications Fig. 7 shows an example of scattering of the evanescent wave in the PSTM from polystyrene particles on quartz. The wave reflected from each particle interferes with the incident wave to produce a standing wave that is m a p p e d by the PSTM. We are currently reflecting the beam in the hemicylinder at the exit face to set up a standing wave at the sample. In this instance the perturbation on the pattern caused by scattering from spheres, or spheroids can be better observed. Also, we are preparing slits on quartz slides to enable observation of diffraction patterns and interference patterns. By these methods we can directly observe electromagnetic scattering p h e n o m e n a from a variety of objects at different wavelengths, polarizations, and angles of incidence.

T.L. Ferrell et al. / Progress in photon scanning tunneling microscopy

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Fig. 7. PSTM image of standing wave pattern produced by backscattering of the evanescent wave from polystyrene markers on quartz.

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Fig. 8 shows how the PSTM has been used to probe the energy density in an optical waveguide [7]. This application may prove to be a major one

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Raman Shift (cm "1) Fig. 10. PSTM-acquired SERS signal from benzoic acid on a silvered crossed-grating.

Fig. 11. Photolithographically produced isolated silver particles on quartz as imaged by the PSTM. The image represents a scan of 0.9 X 0.9 p.m.

T.L. Ferrell et al. / Progress in photon scanning tunneling microscopy

as optical circuits are increasingly used in the telecommunications field. Fig. 9 shows the energy density as a function of distance across one channel of the waveguides. Fig. 10 shows the PSTM-acquired SERS signal from benzoic acid spin-coated onto a silvered crossed-grating. Currently we are attempting to obtain SERS from samples such as shown in the PSTM image of fig. 11. This sample consisted of a rectangular array of isolated silver particles on quartz [5]. We have recently been able to use the same spectroscopic system to detect fluorescence from DNA base pairs in electrophoretic gels and are now awaiting samples with less large-scale roughness in order to permit practicable imaging with the PSTM. The PSTM photolithography results discussed in the previous section are currently being extended by modifying the scan software. These results, and the extension of the PSTM into the soft X-ray spectrum, are priorities for future work. Finally, we have recently been able to obtain some transparent tips for atomic force mi-

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croscopy and have coupled the tips to a fiber in order to combine a PSTM and AFM. This provides a high-resolution optical spectroscopy capability in AFM work.

References [1] R.C. Reddick, R.J. Warmack and T.L. Ferrell, Phys. Rev. B 39 (1989) 767, and US Patent 5018865 (Nov. 1986). [2] T.L. Ferrell, J.P. Goudonnet, R.C. Reddick, S.L, Sharp and R.J. Warmack, J. Vac. Sci. Technol. B 9 (1991) 525. [3] Spiral R&D, 3 Rue des Mardor, Couternon 21560 Arc Sur Tille, France. [4] Dubbeldee Harris Corp., 100 Stierli Court, Mount Arlington, NJ 07856, USA. [5] S.L. Sharp, presented as Poster 2H/87 at: STM '91, Interlaken, 1991. [6] M.A. Paesler, P.J. Moyer, C.J. Jahncke, C.E. Johnson, R.C. Reddick, R.J. Warmack and T.L. Ferrell, Phys. Rev. B 42 (1990) 6750. [7] D.P. Tsai, H.E. Jackson, R.C. Reddick, S.H. Sharp and R.J. Warmack, Appl. Phys. Lett. 56 (1990) 1515.