Ultrafast lasers (picoseconds or femtoseconds) are increasingly used in film pattern processing for the development and production of microelectronics and nanoelectronics devices. Its product applications include photovoltaic cells, displays, sensors, or large-format organic electronic products. The main advantages of ultrafast lasers include limited thermal effect and fast energy dissipation, which helps to realize the pattern processing of complex ultra-thin multilayer film structures.
The advent of the era of nanomaterials provides new processing possibilities for extremely high-speed, high-efficiency and miniaturized equipment. However, processing such new nanomaterials with thicknesses as low as a single atomic layer is technically extremely challenging. This article describes the application of ultrafast lasers for the color processing of atomic-level two-dimensional carbon lattices, namely graphene.
Graphene and laser radiation
In the past ten years, graphene has attracted a lot of attention due to its unique properties and its application in various fields including photovoltaic cells, optoelectronics, sensors, chemical reactions, and energy storage. The industry has successively developed a variety of graphene-based technologies based on traditional methods such as silicon microelectronics. Laser processing has just begun to be used in the development of graphene equipment, but it has shown great potential. Laser beams can be used to perform various treatments on graphene, including laser-assisted graphene growth and pattern ablation on different substrates.
Ultrafast lasers can use a single-step, direct-write laser process to replace the multi-step photolithography process. This is a vital and extremely beneficial process to avoid any impurities formed on the graphene surface due to wet processing.
Graphene pattern ablation
Although the thickness is only as thick as one or a few atomic monolayers, the light absorption rate of graphene is relatively high in a wide electromagnetic spectrum window. For single-layer suspended graphene, the accurate measurement value of visible light is 2.3%. In addition, depending on the properties of the substrate and the bonding surface, the absorptivity of graphene on a specific substrate can be even 10 times higher. When using ultrafast lasers with high photon density, the absorption rate can be further improved.
Figure 1: An example of laser ablation of large-scale graphene patterns.
This provides the possibility for precise and efficient laser ablation of graphene (Figure 1). Electronic applications often require graphene to be placed on thermally grown silicon oxide on top of a silicon substrate. In this structure, the high-efficiency absorption performance of graphene ensures that the graphene can be processed by laser ablation without damaging silicon or silicon oxide.
Since the thickness of graphene is at the atomic level, it is possible to use a single-shot ablation method to shorten the total processing time. Feature sizes of 1 μm or even thinner can be obtained, and laser-induced multiphoton processing can be used to achieve sub-wavelength resolution.
The photochemistry of graphene
The photochemical processing of the material surface is a well-known method. Under ultraviolet light radiation, due to the internal phase shift or the reaction with the surrounding environment (gas, vapor and liquid), the material properties will change. The most common application that utilizes the photochemical properties of laser processing is the additive manufacturing process of multiphoton polymerization using laser radiation. It provides unique processing tools for 3D chemical processing of polymers and composites. The same is true for carbon-based graphene that can also be chemically modified by strong UV oxidation.
Graphene is a unique material regardless of its electronic properties or optical properties. Graphene has verified nonlinear optical effects, such as multiphoton absorption, plasma generation (plasma is the collective excitation of electronic “fluids” in conductive materials), Q-switching, etc. By exploring these nonlinear optical effects, it is expected that high-intensity visible light can be used to change the chemical and optical properties of graphene. Figure 2 shows a typical reaction of local oxidation of graphene using a 515nm ultrafast laser in an oxygen/water atmosphere.
Figure 2: Electron micrograph of graphene oxidation stripes.
The result is that it can produce a free structure with sub-micron resolution (no trace) in a high-speed processing method (with a traditional optical scanner at a processing speed of up to several meters per second). It has surface characteristics such as extreme switching and conductivity difference, obtaining light maneuverability and wettability. This result is very useful, and can quickly develop a variety of equipment or devices used in the biological, security or communication fields.
The various technical characteristics of graphene far surpass the traditional solid-state materials used in electronics, micro-electromechanical systems (MEMS) and micro-opto-electromechanical systems (MOEMS) today. These new features need to be further explored to enable the use of laser processing to obtain technologies with larger scale, faster speed, higher reproducibility, and better purity in order to integrate graphene into new microelectronic platforms.
