Thermal scanning probe lithography used for pattern generation

Thermal scanning probe lithography used for pattern generation
© PiBond / SwissLitho

Thermal scanning probe lithography has been used to create patterns with sub-20nm half pitch resolution. Pattern generation uses thermally sensitive resist materials and spin coatable hard mask materials to transfer resist patterns.

NanoFrazor lithology, also termed thermal scanning probe lithography (tSPL), is based on heated atomic force microscope (AFM) tips to create lithographic patterns by locally evaporating a thermal sensitive resist.

This process enables the fabrication of precise 3D patterns and high-resolution structures without the use of charged particles, such as electrons, which have been implicated in substrate damage. Additionally, AFM imaging is exploited for the in-situ inspection of the substrate surface before patterning and, moreover, for the inspection of the lithographic structures written into the substrate.

Immediate visual feedback of the patterning success allows for fast processing which is further enhanced by the absence of proximity effects. Thermal scanning probe lithography has been established at IMB Research, Zurich, over the past ten years, and in 2012 an independent company SwissLitho AG was founded to commercialise the technology.

Hard-mask pattern transfer

High resolution lithography requires the use of resist layers – typically 10nm or less – which limits the depth of the tSPL written patterns. For real world applications, the depth of these shallow resist patterns needs to be amplified at the device level.

A two-step etch-transfer process involving an extremely thin inorganic hard-mask layer and a thicker organic resist mask has been developed for the transfer of high resolution tSPL resist patterns.

Felix Holzner, CEO of SwissLitho, said: “The Si [silicon] hard mask developed by PiBond is an extremely important step forward, significantly simplifying the hard-mask pattern transfer scheme for our customers.”

In a first step, the shallow tSPL lithographic pattern is transferred into the inorganic hard-mask by means of reactive ion etching (RIE) – silicon dioxide- (SiO2) rich inorganic hard masks with a thickness of only 2-3nm have been found to be ideal for this process yielding high resolution and low line edge roughness patterns;

The thin hard-mask layer is sufficient for the subsequent RIE transfer into the second much thicker organic mask layer. A wide range of carbon-based resists compatible with typical industry processes including metal lift-off can be used for the organic mask.

Fabrication of the extremely thin SiO2 rich hard-mask layer in a uniform and pin-hole free manner is crucial for the transfer process to be successful at sub-20nm half-pitch resolution.

SwissLitho and PiBond have jointly developed a Si hard mask and a process to solve this problem in a way that requires only a simple spin coater for the reliable fabrication of the thin inorganic hard mask layer.

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