Thermal scanning probe lithography has received more research attention in recent years owing to its capacity to direct-write pattern development with nanoscale resolution. These properties have been explored for archival data storage, mask less lithography, and functional templates pattering. The method depends on a nanoindent formation by the self-amplified depolymerization of a poly(phthalaldehyde) thin film in contact with a resistively heated probe.
Thermal scanning probe lithography offers fabrication of 2D and 3D nanopatterns with the same throughput and with a 1nm vertical accuracy. Unfortunately, the maximum depth of the written thermal scanning probe lithography pattern in the resist is limited by the polymer thermomechanical response as well as probe geometry and not exceeding 100nm. In order to get applications that transcend information storage as well as flat surface templates, the depth of the resulting structure must considerably surpass the value. Above all, the chemical and physical attributes of the written pattern must be tuned and tailored properly.
The use of hard mast method is an appealing approach for overcoming the depth limits of thermal scanning probe lithography. It provides pattern amplification in the vertical direction. To produce high aspect ratio nanoscale-to-microscale thermal scanning probe lithography patterns of high precision, the poly(phthalaldehyde) etch attributes play an important role. Unfortunately, no such results have been reported in literature.
Yuliya Lisunova and colleagues at École polytechnique fédérale de Lausanne (EPFL) in collaboration with SwissLitho AG in Switzerland presented a study on the etch properties, particularly the etch selectivity of silicon dioxide, of the poly(phthalaldehyde) thin films with respect to varying fluorocarbon plasma together with the thermal stability of the polymer using inductively coupled plasma source. Their research work is published Microelectronic Engineering.
The research team studied the etch attributes of the film and thermal scanning probe lithography pattern transfer into silicon applying silicon oxide hard mask. In order to obtain a high precision pattern transfer at high amplification factor, the authors analyzed first the etch attributes of the poly(phthalaldehyde) films. They observed that heat treatment by a pre-bake step at temperature below of poly(phthalaldehyde) thin films decomposition considerably improved the it etch resistance, which allowed for a high etch selectivity of silicon and silicon oxide of the heat-treated poly(phthalaldehyde) to fluorocarbon plasma. It allowed for low surface roughness modification and had an etch rate similar to the reference resist.
In view of the results, the authors successfully fabricated 240nm wide line patterns that were etched about 4µm deep into silicon applying as a preliminary layer only a 38nm deep poly(phthalaldehyde) pattern and a hard mask of a 100nm in thickness. The cross-section analysis revealed that the dry etched silicon patterns had vertically sidewall profiles.
The results of Yuliya Lisunova and her colleagues indicate that from an initial shallow poly(phthalaldehyde) pattern made by thermal scanning probe lithography the authors could transfer the pattern by a selected pre-bake process as well as optimized dry etching conditions into deep single crystal silicon. This new nano- to microfabrication method implementing thermal scanning probe lithography is therefore useful for the production of diffraction gratings, microfluidic devices, and nanoimprint stamps.
Lisunova, M. Spieser, R.D.D. Juttin, F. Holzner, J. Brugger. High-aspect ratio nanopatterning via combined thermal scanning probe lithography and dry etching. Microelectronic Engineering, volume 180 (2017), pages 20–24.Go To Microelectronic Engineering