Armin W. Knoll

Advanced scanning probe lithography

The nanoscale control afforded by scanning probe microscopes has prompted the development of a wide variety of scanning-probe-based patterning methods. Some of these methods have demonstrated a high degree of robustness and patterning capabilities that are unmatched by other lithographic techniques. However, the limited throughput of scanning probe lithography has prevented its exploitation in technological applications. Here, we review the fundamentals of scanning probe lithography and its use in materials science and nanotechnology. We focus on robust methods, such as those based on thermal effects, chemical reactions and voltage-induced processes, that demonstrate a potential for applications.

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 ultrahigh-density storage technology

Ultrahigh storage densities can be achieved by using a thermomechanical scanning-probe-based data-storage approach to write, read back, and erase data in very thin polymer films. High data rates are achieved by parallel operation of large two-dimensional arrays of cantilevers that can be batch fabricated by silicon-surface micromachining techniques. The very high precision required to navigate the storage medium relative to the array of probes is achieved by microelectromechanical system (MEMS)- based x and y actuators. The ultrahigh storage densities offered by probe-storage devices pose a significant challenge in terms of both control design for nanoscale positioning and read-channel design for reliable signal detection. Moreover, the high parallelism necessitates new dataflow architectures to ensure high performance and reliability of the system. In this paper, we present a small-scale prototype system of a storage device that we built based on scanning-probe technology. Experimental results of multiple sectors, recorded using multiple levers at 840 Gb/in2 and read back without errors, demonstrate the functionality of the prototype system. This is the first time a scanning-probe recording technology has reached this level of technical maturity, demonstrating the joint operation of all building blocks of a storage device.

Phase behavior in thin films of cylinder-forming ABA block copolymers: Experiments

We experimentally establish a phase diagram of thin films of concentrated solutions of a cylinder forming polystyrene-block-polybutadiene-block-polystyrene triblock copolymer in chloroform. During annealing the film forms islands and holes with energetically favored values of film thickness. The thin film structure depends on the local thickness of the film and the polymer concentration. Typically, at a thickness close to a favored film thickness parallel orientation of cylinders is observed, while perpendicular orientation is formed at an intermediate film thickness. At high polymer concentration the cylindrical microdomains reconstruct to a perforated lamella structure. Deviations from the bulk structure, such as the perforated lamella and a wetting layer are stabilized in films thinner than approximately 1.5 domain spacings.

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.

Effect of confinement on the mesoscale and macroscopic swelling of thin block copolymer films.

We report on the swelling behavior and the corresponding morphological behavior of cylinder-forming polystyrene-b-polybutadiene diblock copolymers, which are confined to several layers of structures. The equilibration of thin films has been done under controlled atmosphere of a nonselective solvent. In situ spectroscopic ellipsometry measurements revealed more than 10% increase of the solvent uptake with decreasing film thickness. With scanning force microscopy of the microphase separated patterns in quenched films, the correlation between the degree of the long-range order of cylinder domains and the degree of the macroscopic swelling has been established. In the case of spontaneously formed micrometer-sized topographic features with discreet film thickness (terraces), the increased solvent uptake by thinner films holds true even for isolated terraces on the mesoscale. The observation of nonhomogeneous swelling of the films on the micrometer scale brings novel insights into the properties of confined soft matter, and suggests new approaches toward the fabrication of polymer-based nanostructured responsive materials.

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.