Nanostencil lithography

Overview

NanostencilThe nanostencil (Fig. 1) is a tool for the fabrication and in-situ characterization of ultraclean nanostructures for fundamental surface science research [1–3]. With this nanostencil technique, the material is evaporated under ultrahigh vacuum (UHV) conditions through a shadow mask directly onto the sample, thereby avoiding the use of photoresists, chemical processing steps, and exposure to air. This allows the fabrication of high-quality structures that are atomically clean, as thin as a few atomic layers, and even epitaxial. Thermal stresses and chemical contamination arising from resist processing are absent. A further advantage of the nanostencil is the easy variability of the materials, i.e., any material prone to evaporation in UHV can be employed, without the need to modify processing steps or masks.

For in-situ characterization by means of scanning probe microscopy (SPM), the carousel (see Fig. 1) is turned to bring an AFM cantilever or STM tip in the position of the fabricated structure (i.e. the former position of the mask).

The different modes of stenciling are static, multistep, and dynamic.

Static mode

In the static mode, structures down to a few tens of nanometers can be fabricated because minimal blurring of the structures is achieved by putting the mask in contact with the sample during evaporation. Figure 2 shows a non-contact atomic force microscopy (nc-AFM) measurement of a test pattern (Cu on SiO2) fabricated in the static mode. 5-nm-thick copper lines with a width of 40 nm are achieved.

Multistep mode

An important advancement is the multistep mode, in which several static evaporation steps are carried out sequentially, allowing the fabrication of multi-mask and multi-material structures. An example is shown in Fig. 3, which displays a pattern for 4-point electrical transport measurements on a nanowire (Cu on silicon oxide). The complete pattern is fabricated using three different masks for (A) contact pads, (B) connection lines, and (C) nanowire, respectively. Between evaporations, the masks are exchanged and translated using the carousel and the linear slider stage, respectively.

Dynamic mode

In the dynamic mode the sample is translated with respect to the mask during evaporation. This provides three major benefits over static stencil lithography, namely, the possibilities to

  • fabricate arbitrary two-dimensional geometries (i.e., a wider range of structures),
  • vary the material thickness (i.e., three-dimensional structures), and
  • stencil different structures using the same mask (flexibility and adaptability).

Figure 4 shows a magnetic nanostructure for the investigation of domain walls in confined geometries [2] (7 nm Fe on silicon oxide) fabricated in the dynamic mode using the mask shown in the inset (silicon nitride membrane structured using a focused ion beam). During evaporation the sample is translated 500 nm in x-direction at constant speed. The structure shows a height modulation of 10%.

Scanning probe microscopy

With the nanostencil tool the fabricated structures are characterized in situ using scanning probe microscopy techniques, i.e. STM (scanning tunnelling microscopy), contact-/ non-contact AFM (atomic force microscopy, see Figs. 2–4) and MFM (magnetic force microscopy). Figure 5 demonstrates submolecular resolution for the nanostencil in nc-AFM. Moreover, scanning probe manipulation techniques will allow further processing of the structures on the atomic/molecular scale.

References

[1] L. Gross, R. R. Schlittler, G. Meyer, and R. Allenspach, Nanotechnology 21, 325301 (2010).

[2] L. Gross, R. R. Schlittler, G. Meyer, A. Vanhaverbeke, and R. Allenspach, Appl. Phys. Lett. 90, 093121 (2007).

[3] P. Zahl, M. Bammerlin, G. Meyer, and R. R. Schlittler, Rev. Sci. Instrum. 76, 023707 (2005).