Technology Introduction¶
Before you start working with the NanoFrazor Explore, please read the brief technology summary below.
Working Principle Quick Summary¶
The technology is a mask-less, serial and direct-write lithography technique, whereby a cantilever with a heatable ultra-sharp silicon tip (which is only a few nanometers in diameter) is brought into contact with a special resist that is spin-coated on a substrate. The horizontal and vertical motion of the substrate/tip can be controlled precisely by means of piezo stages and fast cantilever deflections, allowing sub-20 nm lateral and sub-2 nm vertical resolution. The electrostatic attraction between cantilever and the workpiece bends the cantilever and brings the tip into contact with the resist. At temperatures of up to 1,000 °C the resist (polyphthalaldehyde) evaporates very efficiently and quickly (in less than 2 µs) without residues or contamination, leaving a tiny hole in the resist with the same geometry as the tip. No measurable mechanical force is acting between tip and the resist during the patterning process. This step is repeated at speeds of up to several mm/s in order to create complex 3D surfaces. An important aspect of the technology is the in-situ metrology during the writing process: With the tip in its cold state, the topography of each written line is read back by a second micro-heater which reads the distance to the surface by means of heat conduction through the air. The measured sub-nm deviations from the target are then used as feedback for the running patterning process (a technique called Closed Loop Lithography). Thus, accurate control with unmatched precision of the patterning depth is achieved.
Heatable Cantilevers¶
The heatable cantilevers of the NanoFrazor Explore are made from highly
doped silicon. They have a sharp tip and two heaters. The
heaters are zones with lower doping in the cantilever legs
(figure_cantilever
) a) which are heated by resistive
heating. The first heater, close to the tip, is used to heat up the
tip itself; the second, on the reader leg, is used as a thermal proximity
sensor. The currents flowing through the two heaters can be controlled
independently. In this manual, these two heaters are called writer
and reader, respectively. Electron microscope images of the cantilever
and the sharp tip are displayed in in (figure_cantilever
b)
Positioning¶
The NanoFrazor Explore positioning system is composed of two stages: A fine positioning stage, with sub-nanometer precision control in three axes (XYZ) and a coarse positioning stage which allows for a movement of the cantilever in a range of 10 cm x 10 cm in XY and 10 mm in Z.
During reading and writing, the sample is moved below the writing tip in a raster scan manner by means of the high speed piezo stage. During high speed writing, the fast scanning axes can move with speeds up to 1 mm/s. In order to preserve a high positioning accuracy at such speeds, a feed-forward correction of the input trajectory is performed. The optimized input trajectory is determined prior to all writing or reading operation.
Temperatures¶
The reader and writer temperatures are determined by resistive heating. At low currents and temperatures, silicon is in its extrinsic regime and the resistance increases with temperature. At higher currents and higher temperatures, this semiconductor transitions to its intrinsic regime, and the resistance drops. The temperature at which the maximum of the resistance occurs is a known function of the doping level. The so-called IV curves of the heaters thus enable to perform a temperature calibration. This calibration does not correspond to the temperature at the tip apex, neither in nor out of sample contact. The tips temperature is lower than the heater temperature and strongly depends on the particular substrate and the tip shape.
Height Calibration and Level Plane¶
The height calibration is achieved by approaching the heater toward the sample surface while monitoring the reader resistance and relating it to the positioning signal of the Z piezo scanner. The reader temperature changes because it is cooled by the air by the appropaching the substrate. The signal reaches a constant value as soon as the tip touches the surface and the approach is stopped. This height (where the tip snaps to the surface) defines the zero position of the Z direction.
The level plane adjustment compensates for slightly tilted or curved substrates. The inclination is determined by measuring the distance between the cantilever and the surface, while moving the substrate with the fine positioning stage in both X and Y directions. During reading and writing, the Z piezo stage maintains the same distance during the scanning process by moving the whole cantilever in order to compensate for the measured deviations.
Reading¶
The nano-scale topography is measured in contact mode. As the reader is mechanically connected to the tip, it follows its movement. If the reader comes closer to the surface, or further away from it, the cooling by the substrate through the air increases or decreases, yielding a change in reader resistivity. A measurement of the resistance as the tip scans the surface provides a topography image of the substrate.
Patterning with the Thermosensitive polymer¶
The standard resist for the NanoFrazor Explore is Polyphthalaldehyde (PPA). PPA is a so-called self-amplified unzip polymer which instantaneously decomposes into its volatile monomers upon heat. The unzipping is an endothermic reaction confining the spread of heat in the resist, enabling high-resolution patterning. The decomposition products evaporate immediately without re-deposition on the substrate. Patterned PPA does not need wet chemical development and due to its high glass-transition temperature, it can directly be used as an etch-mask with a high mechanical stability.
The cantilever itself is actuated with an electrostatic, capacitive force by applying voltage on the substrate, pulling the tip towards it. When the hot tip comes into contact with the resist, it evaporates. The evaporation process leaves an indentation of the same outline as the tip, whereby the depth of each indentation can be independently controlled by the electrostatic actuation force with nanometer precision.
Closed Loop Lithography¶
Closed Loop Lithography (CLL) is a combined write-read algorithm and procedure enabling a high degree of depth control. In this mode, at least one read line follows each write line. The key point in CLL is that any deviation from the target depth are instantaneously detected, and can be corrected subsequently.
References¶
The references below give you a good overview on the technology underlying the NanoFrazor Explore.
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- Knoll, A., Rawlings, C., Spieser, M. & Duerig, U. Etch transfer into silicon of patterns with a half-pitch of under 20nm. in SPIE Newsroom (2016).
- Rawlings, C. et al. Accurate Location and Manipulation of Nanoscaled Objects Buried under Spin-Coated Films. ACS Nano 9, 6188–6195 (2015).
- Rytka, C., Kristiansen, P. M. & Neyer, A. Iso- and variothermal injection compression moulding of polymer micro- and nanostructures for optical and medical applications. J. Micromech. Microeng. 25, 65008–65023 (2015).
- Wolf, H. et al. Sub-20 nm silicon patterning and metal lift-off using thermal scanning probe lithography. J.Vac. Sci. Technol. B 33, 02B102 (2015).
- Ristic, S., Nannini, M., Paul, P., Holzner, F. & Grutter, P. Fabrication of Regular and Bi-Level Grating Fiber Couplers using Thermal Scanning Probe Lithography. in OSA Technical Digest (online) IT2A.4 (OSA, 2015).
- Neuber, C. et al. Tailored molecular glass resists for scanning probe lithography. in Proceedings of the SPIE 9425, 94250E–94250E–7 (2015).
- Rawlings, C., Duerig, U., Hedrick, J., Coady, D. & Knoll, A. W. Nanometer Accurate Markerless Pattern Overlay Using Thermal Scanning Probe Lithography. IEEE Trans. Nanotechnol. 13, 1204–1212 (2014).
- Garcia, R., Knoll, A. W. & Riedo, E. Advanced scanning probe lithography. Nat. Nanotechnol. 9, 577–587 (2014).
- Rawlings, C., Duerig, U., Hedrick, J., Coady, D. & Knoll, A. Nanometer control of the markerless overlay process using thermal scanning probe lithography. in 2014 IEEE/ASME International Conference on Advanced Intelligent Mechatronics (AIM) 1670–1675 (2014).
- Cheong, L. L. et al. Thermal Probe Maskless Lithography for 27.5 nm Half-Pitch Si Technology. Nano Lett. 13, 4485–4491 (2013).
- Paul, P., Knoll, A. W., Holzner, F. & Duerig, U. Field stitching in thermal probe lithography by means of surface roughness correlation. Nanotechnology 23, 385307 (2012).
- Holzner, F. et al. Directed Placement of Gold Nanorods Using a Removable Template for Guided Assembly. Nano Lett. 11, 3957–3962 (2011).
- Holzner, F. et al. High density multi-level recording for archival data preservation. Appl. Phys. Lett. 99, 023110 (2011).
- Paul, P. C., Knoll, A. W., Holzner, F., Despont, M. & Duerig, U. Rapid turnaround scanning probe nanolithography. Nanotechnology 22, 275306 (2011).
- Knoll, A. W. et al. Probe-Based 3-D Nanolithography Using Self-Amplified Depolymerization Polymers. Adv. Mater. 22, 3361–3365 (2010).
- Pires, D. et al. Nanoscale Three-Dimensional Patterning of Molecular Resists by Scanning Probes. Science 328, 732–735 (2010).
- Coulembier, O. et al. Probe-Based Nanolithography: Self-Amplified Depolymerization Media for Dry Lithography. Macromolecules 43, 572–574 (2010).
- Knoll, A., Rothuizen, H., Gotsmann, B. & Duerig, U. Wear-less floating contact imaging of polymer surfaces. Nanotechnology 21, 185701 (2010).