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
[ More ]
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
[ More ]
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
[ More ]
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
[ More ]
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
[ More ]
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
[ More ]
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
[ More ]
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
[ More ]
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
[ More ]
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
[ More ]
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.
