l Control on the nanoscale, nanofabrication, nanoscale devices, nano-objects in fluids, IBM Research Zurich

Control on the nanoscale

Nanofabrication, nanoscale devices, nano‑objects in fluids

Overview

Nanoscale patterning is being explored to create unique and novel devices and concepts. We are examining novel fabrication schemes and using their capabilities to create unique functional devices. Applications range from the fabrication of nanoscale electronic devices, (nano-)fluidic concepts for particle placement, transport and separation, to novel devices for studying graphitic interfaces.

Focus areas

Thermal scanning probe lithography

A hot tip is used to produce patterns with sub‑10‑nm lateral and sub‑1‑nm vertical accuracy.

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Nanochisel

Control of objects in nanofluidic confinement

We use geometric nanoscale confinement to create an energy landscape for particles and control their behavior.

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Particle assembly

New active colloidal materials are prepared by a well-controlled sequential assembly process.

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Particle assembly

Thermal scanning probe lithography

t-SPL

Being able to create nanometer accurate patterns and structures is at the heart of nanoscale science and technology. We use a heated scanning probe tip to trigger locally the decomposition reaction of a thermally sensitive resist material. With each contact of the hot tip, a well-defined void is created, resulting in a pattern with high accuracy of its lateral and vertical dimensions. For 2D fabrication, we achieve <10 nm lateral resolution in the resist without it suffering from proximity effects. Linear speeds of up to 20 mm/s and pixel rates of up to 500 kHz have been demonstrated.

In t-SPL, the pattern is created and imaged on the fly, providing direct feedback of the lithography result to the user. The nanometer-precise imaging of the surface prior to patterning enables a sub-5-nanometer precise overlay to existing structures on the sample. Using a dedicated transfer stack, we obtained high-resolution patterns with feature sizes down to 11 nm half pitch in substrate etching, metal lift-off or ion milling. A unique feature of t‑SPL is the capability to write 3D profiles with nanometer accuracy in a single patterning run. We exploit this feature to gain control over objects in nanofluidic confinement.

A unique feature of t‑SPL is the capability to write 3D profiles with nanometer accuracy in a single patterning run.

—IBM scientist Armin Knoll

Fabrication of high-performance solid-state silicon quantum devices

High-resolution patterning with minimal substrate damage

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Fast turnaround fabrication of silicon point-contact quantum-dot transistors using combined thermal scanning probe lithography and laser writing

C.D. Rawlings, Y.K. Ryu, M. Rüegg, N. Lassaline, C. Schwemmer, U. Duerig, A. Knoll, Z.A.K. Durrani, C. Wang, D. Liu, and M.E. Jones
Nanotechnology 29, 505302 (2018).
The fabrication of high-performance solid-state silicon quantum devices requires high-resolution patterning with minimal substrate damage. We have fabricated room temperature (RT) single-electron transistors (SETs) based on point-contact tunnel junctions using a hybrid lithography tool capable of both high resolution thermal scanning probe lithography and high throughput direct laser writing. The best focal z-position and the offset of the tip and the laser-writing positions were determined in situ with the scanning probe. We demonstrate <100 nm precision in the registration between the high-resolution and high-throughput lithographies. The SET devices were fabricated on degenerately doped n-type >1020/cm3 silicon on insulator chips using a CMOS compatible geometric oxidation process. The small size and strong localisation of electrons on the QDs facilitated SET operation even at RT.

Stabilization and control of topological magnetic solitons

Magnetic nanopatterning of exchange bias systems

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Stabilization and control of topological magnetic solitons via magnetic nanopatterning of exchange bias systems

E. Albisetti, A. Calò, M. Spieser, A.W. Knoll, E. Riedo and D. Petti
Appl. Phys. Lett. 113, 162401 (2018).
Stabilizing and manipulating topological magnetic quasiparticles in thin films is of great interest for potential applications in data storage and information processing. Here, we present a strategy for stabilizing magnetic vortices and Bloch lines with controlled position, vorticity, and chirality in a continuous exchange bias system. By tailoring vectorially the unidirectional anisotropy of the system at the nanoscale, via thermally assisted magnetic scanning probe lithography, we show experimentally and via micromagnetic simulations the non-volatile creation of vortex-antivortex pairs. In addition, we demonstrate the deterministic stabilization of cross and circular Bloch lines within patterned Neel magnetic domain walls. This work enables the implementation of complex functionalities based on the control of tailored topological spin-textures in spintronic and magnonic nanodevices.

Modelling the thermo-electrical properties of a complex silicon cantilever structure

Thermal scanning probe lithography

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Comprehensive modeling of Joule heated cantilever probes

M. Spieser, C. Rawlings, E. Lörtscher, U. Duerig and A. W. Knoll
J. Appl. Phys. 121, 174503 (2017).
The thermo-electrical properties of a complex silicon cantilever structure used in thermal scanning probe lithography are modeled based on well established empirical laws for the thermal conductivity in silicon, the electrical conductivity in the degenerate silicon support structure, and a comprehensive physical model of the electrical conductivity in the low-doped heater structure. Excellent agreement between predicted and measured data in the absence of air cooling is obtained if a tapered doping profile in the heater is used. The heat loss through the surrounding air is also studied in a parameter free three-dimensional simulation. The simulation reveals that the heater temperature can be accurately predicted from the electrical power supplied to the cantilever via a global scaling of the power in the power-temperature correlation function, which can be determined from the vacuum simulation.

Thermal scanning probe lithography

Directed self-assembly of block copolymers

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Thermal scanning probe lithography for the directed self-assembly of block copolymers

S. Gottlieb, M. Lorenzoni, L. Evangelio, M. Fernández-Regúlez, Y. Ryu, C. Rawlings, M. Spieser, A. Knoll and F. Perez-Murano
Nanotechnology 28, 175301 (2017).
Thermal scanning probe lithography (t-SPL) is applied to the fabrication of chemical guiding patterns for directed self-assembly (DSA) of block copolymers (BCP). The two key steps of the overall process are the accurate patterning of a poly(phthalaldehyde) resist layer of only 3.5 nm thickness, and the subsequent oxygen-plasma functionalization of an underlying neutral poly(styrene-random-methyl methacrylate) brush layer. We demonstrate that this method allows one to obtain aligned line/space patterns of poly(styrene-block-methyl methacrylate) BCP of 18.5 and 11.7 nm half-pitch. Defect-free alignment has been demonstrated over areas of tens of square micrometres. The main advantages of t-SPL are the absence of proximity effects, which enables the realization of patterns with 10 nm resolution, and its compatibility with standard DSA methods.

High-resolution lithography involving thin resist layers

A challenge for pattern characterization

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Sub-10 Nanometer Feature Size in Silicon Using Thermal Scanning Probe Lithography

Y.K. Ryu Cho, C.D. Rawlings, H. Wolf, M. Spieser, S. Bisig, S. Reidt, M. Sousa, S.R. Khanal, T.D. Jacobs and A.W. Knoll
ACS Nano 11, 11890-11897 (2017).
High-resolution lithography often involves thin resist layers, which pose a challenge for pattern characterization. Direct evidence that the pattern was well-defined and can be used for device fabrication is provided if a successful pattern transfer is demonstrated. In the case of thermal scanning probe lithography (t-SPL), highest resolutions are achieved for shallow patterns. In this work, we study the transfer reliability and the achievable resolution as a function of applied temperature and force. Pattern transfer was reliable if a pattern depth of more than 3 nm was reached and the walls between the patterned lines were slightly elevated. Using this geometry as a benchmark, we studied the formation of 10–20 nm half-pitch dense lines as a function of the applied force and temperature. We found that the best pattern geometry is obtained at a heater temperature of ∼600°C, which is below or close to the transition from mechanical indentation to thermal evaporation. At this temperature, there still is considerable plastic deformation of the resist, which leads to a reduction of the pattern depth at tight pitch and therefore limits the achievable resolution. By optimizing patterning conditions, we achieved 11 nm half-pitch dense lines in the HM8006 transfer layer and 14 nm half-pitch dense lines and L-lines in silicon. For the 14 nm half-pitch lines in silicon, we measured a line edge roughness of 2.6 nm (3σ) and a feature size of the patterned walls of 7 nm.

Control of the interaction strength of photonic molecules

Nanometer-precise 3D fabrication

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Control of the interaction strength of photonic molecules by nanometer-precise 3D fabrication

C.D. Rawlings, D. Urbonas, J. Brugger, M. Spieser, M. Zientek, R.F. Mahrt, T. Stöferle, U. Duerig, Y. Lisunova and A.W. Knoll
Sci. Rep. 7, 16502 (2017).
Applications for high resolution 3D profiles, so-called grayscale lithography, exist in diverse fields such as optics, nanofluidics and tribology. All of them require the fabrication of patterns with reliable absolute patterning depth independent of the substrate location and target materials. Here we present a complete patterning and pattern-transfer solution based on thermal scanning probe lithography (t-SPL) and dry etching. We demonstrate the fabrication of 3D profiles in silicon and silicon oxide with nanometer scale accuracy of absolute depth levels. An accuracy of less than 1 nm standard deviation in t-SPL is achieved by providing an accurate physical model of the writing process to a model-based implementation of a closed-loop lithography process. For transfering the pattern to a target substrate we optimized the etch process and demonstrate linear amplification of grayscale patterns into silicon and silicon oxide with amplification ratios of ∼6 and ∼1, respectively. The performance of the entire process is demonstrated by manufacturing photonic molecules of desired interaction strength. Excellent agreement of fabricated and simulated structures has been achieved.

 

 

 

Control of objects in nanofluidic confinement

Particle–surface interactions in nanofluidic confinement depend strongly on the separation between the surfaces. Accordingly, by shaping the topography of the surfaces, the interaction can be modulated and the particles experience an energy landscape designed by the topography. We strive to use this method to explore novel concepts for the control, transport, separation and positioning of particles.

Current reversal in a rocking Brownian motor

Experimental verification of a longstanding theory

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Experimental Observation of Current Reversal in a Rocking Brownian Motor

C. Schwemmer, S. Fringes, U. Duerig, Y.K. Ryu and A.W. Knoll
Phys. Rev. Lett. 121, 104102 (2018).
A reversal of the particle current in overdamped rocking Brownian motors was predicted more than 20 years ago; however, an experimental verification and a deeper insight into this noise-driven mechanism remained elusive. Here, we investigate the high-frequency behavior of a rocking Brownian motor for 60 nm gold spheres based on electrostatic interaction in a 3D-shaped nanofluidic slit and electro-osmotic forcing of the particles. We measure the particle probability density in situ with 10 nm spatial and 250 µs temporal resolution and compare it with theory. At a driving frequency of 250 Hz, we observe a current reversal that can be traced to the asymmetric and increasingly static probability density at high frequencies.

Nanofluidic rocking Brownian motors

Control and transport of nanoscale objects in fluids

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Nanofluidic rocking Brownian motors

M.J. Skaug, C. Schwemmer, S. Fringes, C.D. Rawlings, A.W. Knoll
Science 359, 1505-1508 (2018).
Control and transport of nanoscale objects in fluids is challenging because of the unfavorable scaling of most interaction mechanisms to small length scales. We designed energy landscapes for nanoparticles by accurately shaping the geometry of a nanofluidic slit and exploiting the electrostatic interaction between like-charged particles and walls. Directed transport was performed by combining asymmetric potentials with an oscillating electric field to achieve a rocking Brownian motor. Using gold spheres 60 nanometers in diameter, we investigated the physics of the motor with high spatiotemporal resolution, enabling a parameter-free comparison with theory. We fabricated a sorting device that separates 60 and 100-nanometer particles in opposing directions within seconds. Modeling suggests that the device separates particles with a radial difference of 1 nanometer.

Nanofluidic confinement apparatus

Studying confinement-dependent nanoparticle behavior and diffusion

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The nanofluidic confinement apparatus: Studying confinement-dependent nanoparticle behavior and diffusion

S. Fringes, M. Skaug and A.W. Knoll
Beilstein J. Nanotechnol. 9, 301-310 (2018).
The behavior of nanoparticles under nanofluidic confinement depends strongly on their distance to the confining walls; however, a measurement in which the gap distance is varied is challenging. Here, we present a versatile setup for investigating the behavior of nanoparticles as a function of the gap distance, which is controlled to the nanometer. The setup is designed as an open system that operates with a small amount of dispersion of ∼20 µL, permits the use of coated and patterned samples and allows high-numerical-aperture microscopy access. Using the tool, we measure the vertical position (termed height) and the lateral diffusion of 60 nm, charged, Au nanospheres as a function of confinement between a glass surface and a polymer surface. We found the height of the particles to be consistently above that of the gap center, corresponding to a higher charge on the polymer substrate. In terms of diffusion, we found a strong monotonic decay of the diffusion constant with decreasing gap distance. For strong confinement of less than 120 nm gap distance, we detect the onset of subdiffusion, which can be correlated to the motion of the particles along high-gap-distance paths.

In situ contrast calibration

Determining the height of individual diffusing nanoparticles in a tunable confinement

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In situ contrast calibration to determine the height of individual diffusing nanoparticles in a tunable confinement

S. Fringes, M. Skaug and A.W. Knoll
J. Appl. Phys. 119, 024303 (2016).
We study the behavior of charged spherical Au nanoparticles in a nanofluidic slit as a function of the separation of the symmetrically charged confining surfaces. A dedicated setup called the nano-fluidic confinement apparatus allows us to parallelize the two confining surfaces and to continuously approach them down to direct contact. Interferometric scattering detection is used to measure the particle contrast with 2 ms temporal resolution. We obtain the confinement gap distance from the interference signal of the glass and the oxide-covered silicon wafer surface with nanometer accuracy. We present a three parameter model that describes the optical signal of the particles as a function of particle height and gap distance.

 

 

Particle assembly

We aim to control the positioning and deposition of colloidal objects that range from several tens of nanometers to a few micrometers in size. Capillary forces at the edge of a moving meniscus are very suitable to position large numbers of colloidal objects in parallel into predefined trapping sites on a template. In a sequential assembly process, new colloidal materials—called “colloidal molecules”—can be produced with an unprecedented freedom of composition and shape. Such colloidal molecules are very promising model systems for molecular assembly processes, and they are building blocks for actively and autonomously moving colloidal objects that can mimic active living matter.

Programmable colloidal molecules

Sequential capillarity-assisted particle assembly

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Programmable colloidal molecules from sequential capillarity-assisted particle assembly

S. Ni, J. Leemann, I. Buttinoni, L. Isa, H. Wolf
Science Advances 2(4), e1501779 (2016).
The assembly of artificial nanostructured and microstructured materials which display structures and functionalities that mimic nature’s complexity requires building blocks with specific and directional interactions, analogous to those displayed at the molecular level. Despite remarkable progress in synthesizing “patchy” particles encoding anisotropic interactions, most current methods are restricted to integrating up to two compositional patches on a single “molecule” and to objects with simple shapes. Currently, decoupling functionality and shape to achieve full compositional and geometrical programmability remains an elusive task. We use sequential capillarity-assisted particle assembly which uniquely fulfills the demands described above. This is a new method based on simple, yet essential, adaptations to the well-known capillary assembly of particles over topographical templates. Tuning the depth of the assembly sites (traps) and the surface tension of moving droplets of colloidal suspensions enables controlled stepwise filling of traps to “synthesize” colloidal molecules. After deposition and mechanical linkage, the colloidal molecules can be dispersed in a solvent. We demonstrate the “synthesis” of a library of structures, ranging from dumbbells and triangles to units resembling bar codes, block copolymers, surfactants, and three-dimensional chiral objects.

Insights into mechanisms of capillary assembly

A powerful method for fabricating complex and programmable particle assemblies

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Insights into mechanisms of capillary assembly

S. Ni, J. Leemann, H. Wolf and L. Isa
Faraday Discuss. 181, 225–242 (2015).
Capillary assembly in a topographical template is a powerful and flexible method for fabricating complex and programmable particle assemblies. To date, very little attention has been paid to the effects that the trap geometry—in particular the trap depth—has on the outcome of the assembly process. In this paper, we provide insights into the mechanisms behind this directed assembly method by systematically studying the impact of the trap depth and the surface tension of the suspension. Using confocal microscopy, we investigate the assembly process at the single-particle level and use these observations to formulate a simple mechanical model that offers guidelines for the successful assembly of single or multiple particles in a trap. In particular, single particles are assembled for shallow traps and moderate surface tensions, opening up the possibility to fabricate multifunctional particle dimers in two consecutive assembly steps.

Capillary assembly of cross-gradient particle arrays

Using a microfluidic chip

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Capillary assembly of cross-gradient particle arrays using a microfluidic chip

S. Ni, M.J.K. Klein, N.D. Spencer, H. Wolf
Microelectronic Engineering 141, 12-16 (2015).
Arrays with well-defined particle registration are of high importance in device fabrication for biosensors, electronics and optics. Also, materials exhibiting gradient variations of properties (e.g. wettability) in one, two or three dimensions have proven their capability for high-throughput screening of various interactions (e.g. cell–surface interactions). Here, we present the fabrication of cross-gradient particle arrays (CGPA) featuring a gradual cross-over from one particle type to another, while keeping the overall particle density constant. CGPAs were prepared by means of a capillary assembly setup assisted by a micro-fluidic chip. This setup offers a high level of control over the capillary assembly process with respect to the composition and location of the assembled arrays. The resulting two complementary gradients may be used in combinatorial studies of biological and chemical interactions. We demonstrate the application of a CGPA as a two-level security feature with unclonable finger-print-like patterns. Moreover, we show the possibility of obtaining a 2D CGPA by capitalizing on diffusive transport within the capillary bridge perpendicular to the assembly direction.

Cascaded assembly of complex multiparticle patterns

A method for the cascaded capillary assembly of different particle populations in a single assembly cycle

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Cascaded Assembly of Complex Multiparticle Patterns

S. Ni, M.J.K. Klein, N.D. Spencer, and H. Wolf
Langmuir 30(1), 90-95 (2014).
A method for the cascaded capillary assembly of different particle populations in a single assembly cycle is presented. The method addresses the increasing need for fast and simple fabrication of multicomponent arrays from colloidal micro- and nanoscale building blocks for constructing nano-electronic, optical, and sensing devices. It is based on the use of a microfluidic device from which two independent capillary bridges extend. The menisci of the capillary bridges are pulled over a template with trapping sites that receive the colloidal particles. We describe the parameters for simultaneous, high-yield assembly from both menisci and demonstrate the applicability of the process by means of the size-selective assembly of particles of different diameters and also by the fabrication of two-component particle clusters with defined shape and composition. This approach allows the fabrication of multifunctional particle clusters having different functionalities at predetermined positions.

Ask the experts

Armin Knoll

Armin Knoll
IBM Research scientist

Heiko Wolf

Heiko Wolf
IBM Research scientist

Projects and collaborations

Projects include an ERC Starting Grant (“Topoplan” no. 307079), an EU project Single Nanometer Manufacturing (SNM), NCCR Molecular Systems Engineering.


Funding by the Swiss National Science Foundation is gratefully acknowledged.


Collaboration partners include the groups of Prof. Elisa Riedo (CUNY NY, USA) and Prof. Lucio Isa (ETHZ).

Selected publications

[1] “Fast turnaround fabrication of silicon point-contact quantum-dot transistors using combined thermal scanning probe lithography and laser writing
C.D. Rawlings et al.
Nanotechnology 29, 505302 (2018).

[2] “Stabilization and control of topological magnetic solitons via magnetic nanopatterning of exchange bias systems
E. Albisetti et al.
Appl. Phys. Lett. 113, 162401 (2018).

[3] “Experimental Observation of Current Reversal in a Rocking Brownian Motor
C. Schwemmer et al.
Phys. Rev. Lett. 121, 104102 (2018).

[4] “Nanofluidic rocking Brownian motors
M.J. Skaug et al.
Science 359, 1505-1508 (2018).

[5] “The Nanofluidic Confinement Apparatus: Studying confinement dependent nanoparticle behavior and diffusion
S. Fringes et al.
Beilstein J. Nanotechnol. 9, 301-310 (2018).

[6] “Comprehensive modeling of Joule heated cantilever probes
M. Spieser et al.
J. Appl. Phys. 121, 174503 (2017).

[7] “Thermal scanning probe lithography for the directed self-assembly of block copolymers
S. Gottlieb et al.
Nanotechnology 28, 175301 (2017).

[8] “Sub-10 Nanometer Feature Size in Silicon Using Thermal Scanning Probe Lithography
Y.K. Ryu Cho et al.
ACS Nano 11, 11890-11897 (2017).

[9] “Control of the interaction strength of photonic molecules by nanometer precise 3D fabrication
C.D. Rawlings et al.
Sci. Rep. 7, 16502 (2017).

[10] “In situ contrast calibration to determine the height of individual diffusing nanoparticles in a tunable confinement
S. Fringes et al.
J. Appl. Phys. 119, 024303 (2016).

[11] “Programmable colloidal molecules from sequential capillarity-assisted particle assembly
S. Ni et al.
Science Advances 2(4), e1501779 (2016).

[12] “Accurate Location and Manipulation of Nanoscaled Objects Buried under Spin-Coated Films
C. Rawlings et al.
ACS Nano 9, 6188-6195 (2015).

[13] “Sub-20 nm silicon patterning and metal lift-off using thermal scanning probe lithography
H. Wolf et al.
J. Vac. Sci. Technol. B 33, 02B102 (2015).

[14] “Insights into mechanisms of capillary assembly
S. Ni et al.
Faraday Discuss. 181, 225–242 (2015).

[15] “Capillary assembly of cross-gradient particle arrays using a microfluidic chip
S. Ni et al.
Microelectronic Engineering 141, 12-16 (2015).

[16] “Advanced scanning probe lithography
R. Garcia et al.
Nat. Nanotech. 9, 577-587 (2014).

[17] “Nanometer Accurate Markerless Pattern Overlay Using Thermal Scanning Probe Lithography
C. Rawlings et al.
IEEE Transactions on Nanotechnology 13, 1204-1212 (2014).

[18] “Cascaded Assembly of Complex Multiparticle Patterns
S. Ni et al.
Langmuir 30(1), 90-95 (2014).

[19] “Thermal probe mask-less lithography for 27.5 nm half-pitch Si technology
L.L. Cheong et al.
Nano Lett. 13, 4485-4491 (2013).

[20] “Nanoscale Contact-Radius Determination by Spectral Analysis of Polymer Roughness Images
A.W. Knoll
Langmuir 29, 13958-13966 (2013).

[21] “Field stitching in thermal probe lithography by means of surface roughness correlation
P. Paul et al.
Nanotechnology 23, 385307 (2012).

[22] “A microfluidic chip setup for capillarity-assisted particle assembly
M.J.K. Klein et al.
Rev. Sci. Instrum. 83(8), 86109, (2012).

[23] “Oriented Assembly of Gold Nanorods on the Single-Particle Level
C. Kuemin et al.
Advanced Functional Materials 22(4), 702–708 (2012).

[24] “Rapid turnaround scanning probe nanolithography
P. Paul et al.
Nanotechnology 22, 275306 (2011).

[25] “Directed Placement of Gold Nanorods Using a Removable Template for Guided Assembly
F. Holzner et al.
Nano Lett. 11, 3957-3962 (2011).

[26] “Probe-Based 3-D Nanolithography Using Self-Amplified Depolymerization Polymers
A.W. Knoll et al.
Adv. Mater. 22, 3361-3365 (2010).

[27] “Nanoscale Three-Dimensional Patterning of Molecular Resists by Scanning Probes
D. Pires et al.
Science 328, 732-735 (2010).

[28] “Wear-less floating contact imaging of polymer surfaces
A. Knoll et al.
Nanotechnology 21, 185701 (2010).

[29] “Probe-Based Nanolithography: Self-Amplified Depolymerization Media for Dry Lithography
O. Coulembier et al.
Macromolecules 43, 572-574 (2009).