While having similar write capabilities as other nanolithography methods in certain aspects, the NanoFrazor adds new possibilities to nanolithography. The following topics explain the unique key features of the NanoFrazor technology in detail.

3D Nanolithography

The unique 3D patterning capabilities of the NanoFrazor are made possible through precise control of the vertical movement of the tip using electrostatic actuation, whereby the actuation force determines the actual penetration depth of the tip into the resist. When this force is varied from pixel to pixel, complex 3D nanopatterns can be created in a single step and with unmatched vertical precision.

In-situ metrology and real-time tuning of patterning parameters

A distinct feature of the NanoFrazor is the capability to simultaneously image the written patterns with high resolution at high speeds. This is accomplished by using micro-heaters which are integrated in the cantilevers. These heaters are confined hot-spots within the silicon having different dopant levels than the rest of the cantilever. During writing, one of the micro-heaters heats up the tip for patterning. For imaging, the cold tip is scanned over the surface while the second micro-heater senses the topography through heat conduction through the air.  Switching between cold and hot tip happens within microseconds, enabling inspection of each written line directly and on-the-fly. Furthermore, this allows automated corrections and real-time adjustments of the patterning parameters, ensuring that the written pattern matches the target pattern. This method, called Closed-Loop Lithography (CLL), provides robustness against external disturbance and guarantees a reliable patterning process. To reduce tip wear, a patented wear-less floating imaging-mode is applied.

No device damage by charged particles

Defects are an issue for standard SEM and e-beam lithography tools as e.g. monolayers of graphene are sensitive to electrons. Even at low energies of about 20 eV, vacancies which reduce the carrier mobility might be created. Similarly, when graphene nanoribbons are imaged at relatively high magnification with a broad electron-beam prior to nanopore creation, their resistance increases linearly with the electron dose, decreasing their conductance and mobility by the same amount. For InAs nanowires, surface charges introduced by electron beam lithography lead to reduced electrical conductivity. In addition, helium ion beam lithography could cause bubble formation and focused ion beams could introduce involuntary implantation, e.g. gallium ions, which cause lattice defects.

Thermal Scanning Probe Lithography as a “beam-free” technique does not induce any device damage by charged particles.

Speed

High-speed patterning using the NanoFrazor is achieved by fast and controlled vertical movement of the tip and the evaporation process of the resist is exceptionally fast and efficient. The NanoFrazor piezo positioning system is specifically designed to cope with the required high scan speeds. A high positioning accuracy and an almost zero out-of-plane movement over the whole scan range guarantee high patterning quality at high speeds.

Apart from writing speed, the overall fabrication time compared to e.g. e-beam lithography can also be shortened with the NanoFrazor technology. Time-consuming, iterative steps like resist development, metrology, OPC are not necessary, which is particularly beneficial for rapid prototyping. Some pattern transfer methods (certain lift-off stacks) do not even require reactive-ion etching, further simplifying processing and decreasing the time required for production.

Resolution

The maximum resolution that the NanoFrazor is capable of achieving is determined by the tip shape – the indents it creates have the size and shape of the tip. The High Power Cantilevers used in the NanoFrazor Explore have radii below 7 nm.

Marker-less overlay

The accurate alignment of lithographic patterns relative to previously written structures (“overlay”) is becoming one of the most critical challenges past the 1x node for today’s nanofabrication tools. The in-situ inspection capability of the NanoFrazor offers an overlay method that does not require sample alignment using markers: Our high accuracy in-situ metrology allows to use already present topographical structures (e.g. electrodes, optical waveguides, nanowires, etc.) from a previous step as a reference for alignment. With that as a reference, high-resolution patterns can be written at the target position and orientation with an overlay error below 5 nm relative to the buried structures.

Correlation stitching

In order to guarantee continuous patterning over areas larger than the piezo scan range, write fields have to be stitched together. The NanoFrazor uses a novel marker-less stitching concept based on the unique surface roughness of the spin-coated polymers. By using the integrated, low-noise in-situ metrology sensor, a correlation of the 2D spectra of the surface roughness before and after moving to the next field is used to determine the movement error which can be determined within 1 nm accuracy. The piezo system is then used to correct for deviations in order to stitch the fields together.

Compatibility with various pattern transfer processes and materials

The patterns written by the NanoFrazor can be transferred into various substrates using standard processing methods like reactive-ion etching and lift-off. The resist that remains after a patterning step serves as an etch mask without the need for wet development. The 2D and 3D topography in the resist can be transferred into materials like silicon using reactive-ion etching with a good etch resistance. To increase the depth amplification and to achieve vertical walls or undercuts, multi-level stacks are used. Transfer processes other than RIE are also possible: For example, materials can be sputtered on top of PPA to form the inverse of the written pattern in the sputtered material. A transferred resolution of 18.3 nm half-pitch by patterning 5 nm into a 9 nm PPA thermal imaging layer has recently been shown.

Feature comparison

A summary of the above chapters of key features, advantages and differences to alternative mask-less lithography techniques is given in the table below.

 DPNaLAObDLWcFIBdEBIDeEBLfNFg
Serial writing processyesyesyesyesyesyesyes
10 nm resolution (half-pitch)noyesnonoyesyesyes
mm/s write speednonoyesnonoyesyes
Standard pattern transfer processes (etching, lift-off, electroplating,...)nonoyesnonoyesyes
Direct write process (no development step)yesyesnoyesyesnoyes
In-situ inspection with < 1 nm vertical resolutionnoyesnonononoyes
3D lithography with < 2 nm vertical resolutionnonononononoyes
Closed-Loop Lithography (combined writing + reading)nonononononoyes
Marker free overlay with < 5 nm accuracynonononononoyes
Stitching using the natural surface roughness as markernonononononoyes
Known to damage materials like graphene, nanowires, etc.nononoyesyesyesno
Proximity corrections necessarynonononoyesyesno
UHV and high voltage necessarynononoyesyesyesno
Total costsmedlowmedhighmedhighmed

a Dip Pen Nanolithography, strength: various bio inks, weaknesses: speed, resolution

b Local Anodic Oxidation (AFM), strength: direct write, weakness: speed

c Direct Laser Writing (photoresist), strength: fast, weaknesses: diffraction limited

d Focussed Ion Beam (Gallium milling), strengths: 3D, all materials, weaknesses: speed, damage

e Focussed Electron Beam Induced Deposition, strength: high resolution, weakness: speed

f Electron Beam Lithography (HSQ or PMMA), strengths: resolution, well established, weaknesses: proximity effect, complexity

g NanoFrazor Explore

Beyond resist-based lithography

Although several applications have already been tried out by a number of research groups, the research field using heated tips at the nanoscale is still quite young and largely unexplored.

The NanoFrazor’s software, electronics and housing are designed to be flexible, permitting adjustments and extensions for applications that go beyond the standard topographical patterning mode. Temperature, contact force and contact time of the tips can be controlled very accurately, making the NanoFrazor technology a versatile tool to study and modify surfaces at the nanoscale over a wide parameter range for various applications and studies beyond lithography.

For example, one can locally vary the temperature with nanometer precision. This can, for example, enable thermal gradients and chemical functionalization of resists as used in Thermochemical Scanning Probe Lithography. Another method is the local reduction of graphene oxide to induce an increase in electrical conductivity by some orders of magnitude. The heat of the tip can also be used to modify polymers to create amine patterns (as for bio-functionalization) to attach various kinds of bio-molecules. Further alternative uses could be thermal imaging when the thermal cantilever measures the temperature distribution on the substrate with nanoscale resolution. Using the NanoFrazor High Power Cantilevers, the hot spots in electronic nanowire devices have been characterized.