Nanostructuring Using Combined Laser / STM (FOLANT Method Offers Unique Capabilities)

Nanostructuring using scanning probe microscopy (SPM) is gaining increased importance. STM nanostructuring, for example, has been easily performed by scratching a substrate with an STM probe tip (1). Nanostructuring has also been performed by superimposing voltage pulses over the regular STM tunneling voltage (2,3). Near-field Scanning Optical Microscopy (NSOM) has also been used successfully for illumination of photoresist with lateral resolution down to 10nm (4).

Clearly, SPM technology is not only important for high resolution imaging, but also for nanolithography. Today, we are in the very early stages of developing this new tool. In this article, we present a new laser / STM nanostructuring method (the "FOLANT" technique) which has been developed in our laboratory for future high density data storage. We anticipate many other promising applications for this technique.

FOLANT utilizes the field enhancement of optical radiation in the near-field of small conductive particles, as has been used for surface enhanced Raman spectroscopy (SERS) since the early 1980s. SERS is based on various electrostatic and electrodynamic effects, such as the lightning-rod effect, and the excitation of plasmon resonances (5).

This effect has recently been transferred to an STM probe tip for applications in nanostructuring (6, 7). The probe tip is illuminated from the side by a laser, as shown in Figure 1. Intensity enhancement of up to 10 x6, depending primarily on the tip material and shape, occurs beneath the tip. The enhancement area is limited to only a few nanometers, thus allowing nanoprocessing of a substrate within the near-field. In contrast to conventional focusing of laser radiation by lenses or mirrors, we named this the "FOLANT Technique" (Focusing of Laser radiation in the Near-field of a Tip).

When performing FOLANT nanostructuring, the feedback control system of the STM (Figure 1) is used only to ensure that the tip and substrate are within a few Angstroms, and thus that the substrate is within the near-field of the tip. The x-y scanning features of the STM are used for generating desired nanostructures. We have developed a complete nano-machining center based on the TopoMetrix Discoverer STM, and a frequency-doubled Nd:YAG laser. The STM is also used for imaging the processed nano-topography.

For our experiments, we used probe tips with radii of less than 50nm which were made from either silver or tungsten (8). The substrates were made from mica, which was coated with a 100nm gold layer with a roughness of less than 3nm. Nanostructuring using the FOLANT technique was performed under ambient conditions.

When silver tips are used, small hillocks are generated on the substrate by each laser pulse. Figure 2 shows several such nano-hillocks which were formed by various laser intensities. The highest is approximately 10nm, with a lateral dimension of 20-40nm. Investigations of the interaction process have shown that the structures were generated by material transfer from the tip to the substrate. Continuous line structures have been produced by forming single hillocks in close proximity.

When tungsten tips are used instead of silver, the interaction process results in holes in the surface. "Nano-holes," with diameters down to 30nm, were produced in the gold layer. Continuous cuts, or trenches with a width of less than 10nm were generated by a sequence of laser pulses and continuous tip scanning (Figure 3). The interaction mechanism is not yet completely understood, but we assume that dipole-dipole interactions between the tip and gold clusters are involved.

Figure 3. Parallel arrangement of 10 nm cuts in gold substrate.

In contrast to other SPM-based nanostructuring methods, the FOLANT technique is characterized by a flexible interaction process. In addition, it can be performed in ambient conditions. Compared to NSOM, the promary advantage is the independence of the laser wavelength and the availability of higher intensity radiation.

Figure 4. Nanostructuring with FOLANT technique. ("DPG" is the abbreviation of the "Deutsche Physikalische Gesellshaft).

High density data storage of up to 10 x12 bit / cm x2 should be possible. Further applications might be found in mask repair systems for the semiconductor industry, processing of Fresnel optics for x-ray focusing, and as a "nano-scalpel" for biotechnology.

(1) D.W. Abraham, H.J. Mamin, E. Ganz, J. Clarke : IBM J. Res. Dev. 30 (1986) Nr. 5, 492

(2) R.M. Silver, E.E. Ehrichs, A.L. Lozanne : Appl. Phys. Lett 51 (1987) Nr. 4, 247

(3) Th. Schimmel, H. Fuchs : Phys. Blatter, 50 (1994), Nr. 6, 5773-5774

(4) D.W. Pohl, R. Wiesendanger, H.J. Güntherodt : Scanning Tunneling Microscopy II, Springer Series in Surface Science 28 (1993)

(5) A. Wokaun : Mol. Phys, 56, 1 (1985)

(6) A.A. Gorbunov, W. Pompe : Phys. Stat. Sol. A 145, 333 (1994)

(7) J. Jersch, K. Dickmann : Appl. Phys. Lett. 68 (6), 5 February 1996

(8) K. Dickmann, F. Demming, J. Jersch : Rev. Sci. Instrum, (to be published)

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