Thermomechanical Nanocutting of 2D Materials

Giovanni Boero, Jürgen Brugger, Ana Conde Rubio, Samuel Tobias Howell, Xia Liu
2020
Article

Résumé

Atomically thin materials, such as graphene and transition metal dichalcogenides, are promising candidates for future applications in micro/nanodevices and systems. For most applications, functional nanostructures have to be patterned by lithography. Developing lithography techniques for 2D materials is essential for system integration and wafer-scale manufacturing. Here, a thermomechanical indentation technique is demonstrated, which allows for the direct cutting of 2D materials using a heated scanning nanotip. Arbitrarily shaped cuts with a resolution of 20 nm are obtained in monolayer 2D materials, i.e., molybdenum ditelluride (MoTe2), molybdenum disulfide (MoS2), and molybdenum diselenide (MoSe2), by thermomechanically cleaving the chemical bonds and by rapid sublimation of the polymer layer underneath the 2D material layer. Several micro/nanoribbon structures are fabricated and electrically characterized to demonstrate the process for device fabrication. The proposed direct nanocutting technique allows for precisely tailoring nanostructures of 2D materials with foreseen applications in the fabrication of electronic and photonic nanodevices.

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https://infoscience.epfl.ch/record/278223?ln=fr

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Thermomechanical nanocutting of 2D materials using thermal scanning probe lithography

Giovanni Boero, Jürgen Brugger, Ana Conde Rubio, Samuel Tobias Howell, Xia Liu

The rapidly evolving field of 2D materials has developed a plethora of emerging micro/nano devices where atomically-thin materials play a fundamental role. In most cases, this implies the need of nanostructuring them, but due to their delicate nature, conventional lithographies using electrons or ions can deteriorate their performance. Thermal scanning probe lithography (t-SPL) is an advanced direct-write method that uses a heated nanotip for 2D and 3D subtractive/additive manufacturing [1]. The creation of patterns by t-SPL is accomplished by consecutive indentation of the sample with the heated nanotip while simultaneously scanning the sample. We extend the application of t-SPL beyond polymers and demonstrate the direct cutting of different 2D materials using a heated scanning nanotip [2]. To do so, a 2D material flake is transferred onto a substrate covered by polyphthalaldehyde (PPA), a dedicated polymer that unzips into smaller volatile monomers at around 150 °C, as shown in Figure 1(a). When the tip indents the 2D material, the applied force, together with the heat, induces the rapid sublimation of the PPA and breaks the chemical bonds of the thin flake without damaging the rest of the material. In this way, we created different test structures in the 2D material, i.e. nanoribbons as shown in Figure 1(b). A resolution down to 20 nm is achieved for monolayered MoTe2; yet many other 2D materials, multilayers, and even heterostructures can be patterned. Raman spectroscopy is used to validate the removal of the 2D materials in the cut areas and electrical characterization shows that clean cuts have been achieved. In the future, this high-resolution nanocutting process can be of benefit to scientists and engineers as a fast prototyping technique to fabricate 2D material nanodevices by a clean, one-step thermo-mechanical technique. References [1] S.T. Howell, A. Grushina, F. Holzner, J. Brugger, Microsyst. Nanoeng.,6 21 (2020). [2] X. Liu, S.T. Howell, A. Conde-Rubio, G. Boero, J. Brugger, Adv. Mater. (2020). doi:10.1002/adma.202001232

2020

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Nanostencil Lithography - Quick & Clean: Towards a reliable scalable nanopatterning method

Jürgen Brugger, Nao Takano, Marc Antonius Friedrich van den Boogaart, Oscar Vazquez Mena

Stencil lithography is a surface patterning technique that relies on the local deposition of material through miniaturized shadow mask membranes. This method has been used since many years in various implementations for the formation of patterns, mainly structured thin metal films, in situations when lithography equipment is not available or when the surfaces don’t allow the harsh process steps typically involved in photolithography such as spin coating of photoresist, high temperature baking, development, wet or dry etching and resist stripping. Stencil lithography is down-sizable into the sub-100-nm scale which makes it very interesting as method for (in-vacuum) rapid-prototyping of nanostructures without the risk of contamination, and for laboratories without access to high-end nanolithography equipment. The major challenges in stencil lithography are following: i) the fabrication of large area nanostencils requires one high-resolution lithography step and sophisticated MEMS processes, ii) the mechanical properties of thin membranes (typically ~ 100 nm thick) requires careful handling, iii) the effect of deposited material on stencil leads to aperture clogging and membrane bending which limits the resolution capabilities, iv) the precise positioning of nanostencils to prefabricated surfaces structures for aligned multiple layer patterning, v) and eventually the recycling of used stencils. Despite these difficulties, stencils are extremely useful tools for flexible and reliable patterning of surface structures across multiple length-scales from mm to sub-100 nm. In collaboration with our project partners we have recently progressed in several of the above mentioned challenges: the fabrication of stencils is now based on a set of advanced silicon micromachining steps including DUV exposure for >200 nm apertures and a combination of DRIE and wet etching. Focused beams are used to fabricate stencils with sub-100 nm apertures. In particular the mechanical stability of the ultra-thin low-stress silicon nitride membranes could be considerably improved by topographic reinforcement rims. As a result, the membranes deform less under the stress of deposited film and consequently the surface patterns are better defined. We have studied patterning by stencil lithography of metals (e.g. Al, Au, Bi, Cr, Ti, Cu) on various surfaces (e.g. Si, SiO2, SU-8, PDMS, PMMA, freestanding SiN cantilevers, and curved surfaces. Pulsed laser deposition of metal on self-assembled monolayers (SAMs) has resulted in novel micro/nanopattern for molecular electronic devices. Parallel fabrication of thousands of sub-micrometer resonating beams that are integrated with CMOS circuitry have been realized by stencil lithography and sacrificial etching. The talk will present the current state-of-the-art of the nanostencil lithography , will highlight the strength of the method but will also discuss the current limits and challenges ahead to make it a truly reliable nanofabrication method.

2006

Chargement

Two-dimensional (2D) materials have received tremendous research attention recently, as they possess peculiar physical properties in their monolayer and few-layer forms, which further lead to novel applications. A wide range of 2D materials covers insulators, metals, superconductors and semiconductors with various bandgap, giving a fresh platform to reinvent conventional electronic and optoelectronic devices with new advantages. Molybdenum disulfide (MoS2), belonging to the group of transition metal dichalcogenides (TMDCs), has been widely studied for its particular physical properties and potential applications as a leading example among 2D semiconductors. In this thesis, we focus on the applications ofMoS2 in microelectronic and micro-electromechanical devices. The MoS2 we studied is grown specifically by chemical vapor deposition, which offers the highest potential in large area growth. Utilizing the flexible nature such as high in-plane fracture strain and low bending stiffness of atomically thinMoS2, we developed stretchable thin film transistors (TFTs) on polydimethylsiloxane (PDMS) and highly bendable thin film transistors on polyimide. The fabrication process is ameliorated and designed to provide good chemical and temperature compatibility to the polymer substrates. Electrical performance of the flexible devices under different degrees of bending and stretching has been studied. The stretchableMoS2 TFTs could sustain up to 4% of uniaxial tensile strain maintaining device function, and the failure occurs at 5% of strain due to the fracture in gate dielectric layer. The highly bendableMoS2 TFTs with parylene-C gate dielectric layer exhibit tolerance to the smallest bending radius of 0.625 mm. Different encapsulation layers such as HfO2, Cytop and SU8 have been implemented on the bendable devices to improve the performance represented by the increase in field effect mobility, reduction in hysteresis and better endurance to mechanical deformation. The resulting flexibleMoS2 TFTs show field effect mobility of 21.4 cm^2V^-1s^-1 and a large on-off ratio of 106 in air. The SU8 encapsulation layer is further demonstrated as controllable n-type doping strategy with low temperature process, which is ideal for flexible MoS2 devices as it additionally exhibits good stability in air and water.

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Nanostencil Lithography - Quick & Clean: Towards a reliable scalable nanopatterning method

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Thermomechanical nanocutting of 2D materials using thermal scanning probe lithography

Giovanni Boero, Jürgen Brugger, Ana Conde Rubio, Samuel Tobias Howell, Xia Liu

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