Urs T. Duerig

Nanoscale Three-Dimensional Patterning of Molecular Resists by Scanning Probes

Patterning a Molecular Glass Lithographic patterning for device fabrication is usually based on initiating polymerization reactions with photons or electrons in a molecular resist. However, patterning can be achieved by mechanically removing a hard resist with scanning probe microscopy tips, but in many cases the resolution is low and excess material is left on the surface. Pires et al. (p. 732, published online 22 April) found that thin films of organic molecules could form glasses through weak interactions and be patterned to tens of nanometers with a heated scanning probe tip. These patterns could be transferred to other substrates or sculpted into three-dimensional shapes by successive rounds of patterning. A molecular glass can be patterned to dimensions of tens of nanometers with a heated scanning probe tip. For patterning organic resists, optical and electron beam lithography are the most established methods; however, at resolutions below 30 nanometers, inherent problems result from unwanted exposure of the resist in nearby areas. We present a scanning probe lithography method based on the local desorption of a glassy organic resist by a heatable probe. We demonstrate patterning at a half pitch down to 15 nanometers without proximity corrections and with throughputs approaching those of Gaussian electron beam lithography at similar resolution. These patterns can be transferred to other substrates, and material can be removed in successive steps in order to fabricate complex three-dimensional structures.

Probe‐Based 3‐D Nanolithography Using Self‐Amplified Depolymerization Polymers

Scanning probes are capable of addressing and modifying surface structures on the atomic scale, [ 1 ] a capability that has been exploited to create molecular logic devices. [ 2 ] However, in realworld applications, the production of nanoscale patterns and devices requires substantial throughput capabilities in combination with suffi cient tip endurance to address areas on the order of 0.1–1 mm 2 at high resolution. At a typical pixel pitch of 10 nm, this translates to 10 8 –10 10 pixels being written with a single tip. Therefore a highly sensitive patterning approach that is gentle on the tip would be indispensable. Besides the well-established method of local anodic oxidation, [ 3 – 5 ] recent developments in this direction are the fi eld-induced deposition of materials [ 6 , 7 ] and the tip-induced modifi cation or removal of thermomechanically responsive organic materials. [ 8 – 12 ] In addition, it has been shown that on polymeric substrates the wear on a sliding silicon tip can be virtually eliminated, [ 13 ] which is a prerequisite for high-resolution patterning on technologically viable scales. Other stimuli have been used to structure polymers locally using atomic force microscopy (AFM) tips, e.g., mechanical forces in plowing [ 14 ] and ultrasonic patterning [ 15 ] or electron irradiation using fi eld emission from the tip. [ 16 ] In this paper, we describe the fabrication of twoand threedimensional structures based on the local removal of a resist polymer using heated tips. Previous experiments have shown that suffi cient energy is provided by heated tips to break the chemical bonds of a Diels–Alder material, [ 11 ] which subsequently decomposes into volatile monomer units. However, the overall patterning effi ciency is low. The effi ciency can be dramatically enhanced by using self-amplifi ed depolymerization (SAD) polymers. Here, the breaking of a single bond induces the spontaneous depolymerization of the entire polymer chain, [ 17 , 18 ] a concept that was fi rst demonstrated in the early 80’s as a dry lithography approach. Recently it was discovered that using phthalaldehyde SAD polymers two-dimensional

Adhesion and friction in mesoscopic graphite contacts

Using friction to guide fabrication Ultralow friction found in certain layered materials such as graphite is important in the construction of nanomechanical devices. Koren et al. combined measurements and modeling to characterize the interaction of sliding graphite planes (see the Perspective by Liechti). This helped them to make small graphite devices that featured rotational pivots and multiple locking positions. Science, this issue p. 679 The measurement and modeling of friction between graphite planes allows for clever engineering of small mechanical devices. [Also see Perspective by Liechti] The weak interlayer binding in two-dimensional layered materials such as graphite gives rise to poorly understood low-friction characteristics. Accurate measurements of the adhesion forces governing the overall mechanical stability have also remained elusive. We report on the direct mechanical measurement of line tension and friction forces acting in sheared mesoscale graphite structures. We show that the friction is fundamentally stochastic in nature and is attributable to the interaction between the incommensurate interface lattices. We also measured an adhesion energy of 0.227 ± 0.005 joules per square meter, in excellent agreement with theoretical models. In addition, bistable all-mechanical memory cell structures and rotational bearings have been realized by exploiting position locking, which is provided solely by the adhesion energy.

Directed placement of gold nanorods using a removable template for guided assembly.

We have used a temperature sensitive polymer film as a removable template to position, and align, gold nanorods onto an underlying target substrate. Shape-matching guiding structures for the assembly of nanorods of size 80 nm × 25 nm have been written by thermal scanning probe lithography. The nanorods were assembled into the guiding structures, which determine both the position and the orientation of single nanorods, by means of capillary interactions. Following particle assembly, the polymer was removed cleanly by thermal decomposition and the nanorods are transferred to the underlying substrate. We have thus demonstrated both the placement and orientation of nanorods with an overall positioning accuracy of ≈10 nm onto an unstructured target substrate.

Thermal Probe Maskless Lithography for 27.5 nm Half-Pitch Si Technology

Thermal scanning probe lithography is used for creating lithographic patterns with 27.5 nm half-pitch line density in a 50 nm thick high carbon content organic resist on a Si substrate. The as-written patterns in the poly phthaladehyde thermal resist layer have a depth of 8 nm, and they are transformed into high-aspect ratio binary patterns in the high carbon content resist using a SiO2 hard-mask layer with a thickness of merely 4 nm and a sequence of selective reactive ion etching steps. Using this process, a line-edge roughness after transfer of 2.7 nm (3σ) has been achieved. The patterns have also been transferred into 50 nm deep structures in the Si substrate with excellent conformal accuracy. The demonstrated process capabilities in terms of feature density and line-edge roughness are in accordance with todays requirements for maskless lithography, for example for the fabrication of extreme ultraviolet (EUV) masks.

Nanoscale shape-memory function in highly cross-linked polymers.

Topographic engraving of structures in polymer surfaces attracts widespread interest for application in imprint lithography and data storage. We study the nonlinear interaction of nanoindents written in close proximity, 20-100 nm, to one another in a highly cross-linked polystyrene matrix. The indents are created thermomechanically by applying heat and force stimuli of 10 micros duration to a tip, thereby raising the polymer temperature to 250 degrees C and exerting contact pressures of up to 1 GPa. We show that on the nanoscale plastic deformation is highly reversible providing outstanding shape-memory functionality of the material.

Curved in-plane electromechanical relay for low power logic applications

A curved design for in-plane micro- and nano-electromechanical switches based on a single clamped cantilever is proposed, optimized with finite-element simulations and demonstrated experimentally. The design enables precise control of the switch motion and of the closed-state air gap, resulting in a uniform electrostatic field and increased robustness. The switch size and curvature are optimized for actuation voltage, actuation energy and the electrostatic field strength. These optimizations and the proposed fabrication process are amenable to micro- and nano-electromechanical switches. The scalability of the concept is demonstrated with simulations of nanoscale relays in terms of force and energy, showing that the concept is suitable for sub-100 aJ switching energy. Experimental results on microscale devices demonstrate the advantages of the curved MEM switches, namely a fabrication process with a single sacrificial layer for a switch with a low actuation voltage and excellent robustness. The designed as well as the experimentally observed breakdown voltage is four times higher than the contact voltage, thus enabling a large operating window for electromechanical switches. (Some figures may appear in colour only in the online journal)

Multi Tbit/in2 Storage Densities with Thermomechanical Probes

Exploiting the spatial resolution of scanning probes presents an attractive approach for novel data storage technologies in particular for large-scale data repositories because of their inherent potential for high storage density. We show that multi-Tbit/in(2) density can be achieved by means of thermomechanically embossing the information as indentation marks into a polymer film. The data density is determined by the nonlinear interaction between closely spaced indents and the fundamental scaling relations governing the shape and size of the indents. We find that cooperative effects in polymers give rise to a minimum indentation radius on the order of the correlation length of the cooperatively rearranged region even if formed by an infinitely sharp indenter. Thus, cooperativity coupled to alpha-transitions in polymers is evinced in a real space geometrical experiment. Furthermore, we predict that indentation marks cannot be made smaller than 5 nm in diameter, which limits the feature resolution for embossing technologies in general.

Coherent commensurate electronic states at the interface between misoriented graphene layers

Graphene and layered materials in general exhibit rich physics and application potential owing to their exceptional electronic properties, which arise from the intricate π-orbital coupling and the symmetry breaking in twisted bilayer systems. Here, we report room-temperature experiments to study electrical transport across a bilayer graphene interface with a well-defined rotation angle between the layers that is controllable in situ. This twisted interface is artificially created in mesoscopic pillars made of highly oriented pyrolytic graphite by mechanical actuation. The overall measured angular dependence of the conductivity is consistent with a phonon-assisted transport mechanism that preserves the electron momentum of conduction electrons passing the interface. The most intriguing observations are sharp conductivity peaks at interlayer rotation angles of 21.8° and 38.2°. These angles correspond to a commensurate crystalline superstructure leading to a coherent two-dimensional (2D) electronic interface state. Such states, predicted by theory, form the basis for a new class of 2D weakly coupled bilayer systems with hitherto unexplored properties and applications.

Ultraflat Templated Polymer Surfaces

The roughness of spin-cast polymer films arises from thermally activated capillary waves during preparation and typically amounts to about 0.5 nm(rms) measured on a micrometer-sized surface area. Templating from atomically flat mica substrates allows the creation of polymer films with a surface roughness approaching the molecular scale. Three regimes of spatial frequencies are identified in which the roughness is controlled by different physical mechanisms. We find that frozen-in elastic pressure waves ultimately limit the flatness of polymer films.