Active-probe AFM across applications
Scanning Probe Lithography
Field Emission based Scanning Probe Nanolithography (FE-SPL) provides a cost-effective approach to reproducible single-digit nanometre patterning for electronic and optical nanodevice fabrication and other areas of nanotechnology. A key advantage of FE-SPL is the use of self-sensing and self-actuating active cantilevers, which combine lithography, AFM imaging and precise positioning within the same scanning probe platform.
FE-SPL uses low-energy electrons emitted from an ultra-sharp probe positioned in close proximity to a resist-covered surface. With electron energies below 100 eV, the lithographic interaction can be confined to a very small volume within ultra-thin molecular resist layers, enabling highly localized positive- and negative-tone patterning while reducing electron penetration, backscattering and proximity effects. This approach has demonstrated single-digit nanometre patterning, including sub-5 nm features under suitable process conditions. For calixarene resist under ambient conditions, approximately 10 electrons per molecule were sufficient to induce crosslinking, corresponding to a sensitivity around 80 times higher than conventional electron-beam exposure at 30 keV. Ambient FE-SPL has also demonstrated a positive-tone patterning mechanism based on direct resist ablation.
Templates for nanoimprint lithography
FE-SPL can be used to fabricate high-resolution master templates for Nanoimprint Lithography (NIL), particularly where the critical features of the master require single-digit nanoscale dimensions. The nanoscale features are written directly using FE-SPL and can then be transferred into a reusable imprint template for subsequent replication using NIL. Depending on the material system, the written pattern can be transferred into the underlying substrate, for example by reactive ion etching, to form a durable relief structure.
This combination addresses two different requirements of nanofabrication: FE-SPL provides the resolution and precise definition of the original nanoscale features, while NIL provides an efficient route for reproducing those features over multiple samples. It is therefore particularly attractive when extremely small critical structures must first be generated with high precision and then reproduced repeatedly without performing the complete direct-write process for every copy.
Room-temperature single-electron transistors
FE-SPL enables a reproducible fabrication route for
room-temperature single-electron transistors (RT-SETs).
It allows the critical nanoscale structures to be patterned
directly with single-digit nanometre resolution. Its ability
to image the surface before and after lithography, together
with precise feature alignment, supports reliable positioning
of the nanoscale device structures relative to existing
electrodes.
The lithographically defined patterns can subsequently be
transferred into the substrate using cryogenic reactive ion
etching, enabling highly anisotropic transfer of
high-aspect-ratio nanostructures. Devices fabricated using
the FE-SPL and cryo-RIE approach have demonstrated
single-electron effects at room temperature, illustrating
its potential for reproducible fabrication of nanoscale
electronic devices beyond conventional CMOS scaling.
Nanopatterning of 2D materials
FE-SPL is used to define nanoscale structures directly in
two-dimensional materials. By locally patterning the material,
features such as narrow channels, constrictions, isolated
regions and other device geometries can be created for
electronic, optoelectronic and nanoscale device research.
FE-SPL offers a highly localized approach to this patterning
task. Its low-energy electron exposure is confined close to
the surface, reducing electron penetration and backscattering
compared with conventional high-energy electron-beam
lithography. At the same time, the scanning probe can be
used for AFM imaging, positioning and lithography, allowing
patterns to be accurately placed and inspected before and
after writing. FE-SPL can also be used to pattern within already structured device layouts, including across existing
contacts and topographical features.
Mix-and-match lithography
FE-SPL can be combined with other lithographic techniques such as electron-beam lithography (EBL) or optical lithography in a mix-and-match fabrication strategy, allowing structures produced by different lithographic methods to be integrated within the same device layout. Larger structures can be patterned using conventional methods while FE-SPL is used for the critical nanoscale features.
A key advantage of FE-SPL in this workflow is its scanning-probe-based imaging and positioning capability. The same probe can locate existing EBL structures, align the FE-SPL pattern to them and inspect the result after writing. This enables highly accurate overlay and stitching between separately fabricated nanoscale features.
The combined pattern can subsequently be transferred into a durable substrate to form a master template for NIL. In this way, mix-and-match lithography can combine efficient fabrication of larger structures with the high-resolution and high-placement-accuracy capabilities of FE-SPL, while NIL provides a route for repeated replication of the completed nanoscale pattern.