Thermal Scanning Probe Lithography – the technology behind the NanoFrazor, was invented at IBM Research in Zurich. The standard process is a subtractive technique where material is selectively removed from a surface and could be described as “inverse 3D printing on the nanoscale”.
The animation below illustrates the patterning principle in slow motion.
This approach is a maskless, direct-write lithography technique. A special, thermally sensitive resist is spin coated onto the sample surface before patterning. A heated ultra-sharp tip is then used to decompose and evaporate the resist locally, thereby creating any arbitrary pattern in the resist. This resist pattern can then be transferred into almost any other material by means of lift-off, etching, plating, molding, etc.
The horizontal and vertical motion of the tip is controlled precisely by means of piezo stages and fast electrostatic cantilever deflections, allowing sub-20 nm lateral and sub-2 nm vertical resolutions. The heated resist (usually polyphthalaldehyde) decomposes into volatile molecules that evaporate, leaving a tiny hole in the resist with the same geometry as the tip. The depth of each hole is precisely defined by the electrostatic bending of the cantilever enabling 3D shapes to be written with unmatched accuracy in just one single step. The evaporation of the resist happens quickly (in less than 2 µs) which allows patterning at tip-scan speeds of several mm/s.
An important aspect of the technology is the in-situ metrology during the patterning process. With the tip in its cold state, the topography of each patterned line is imaged using an integrated topography sensor in the cantilever. The measured sub-nm deviations from the target depth are then used as feedback for the next line of the pattern. Thus, the NanoFrazor can achieve accurate and autonomous control of the patterning depth. This novel approach for lithography, where the written nanostructure is constantly measured during patterning itself is labeled “Closed Loop Lithography”.
As an alternative to the evaporation of resists, the hot tip can also be used to directly trigger any kind of temperature induced modification on the surface. The heater temperature can be precisely controlled up to around 1000°C and heated tips have been used for local chemical modification or phase changes of a surface.
For an overview of such application of the heated tip, have a look at the work of our customer Prof. Elisa Riedo in New York.