Category Archives: Laser Microprocessing

Superhydrophobic and Superhydrophilic Functionalization of Engineering Surfaces by Laser Texturing

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By Suwas Nikumb, Peter Serles, and Evgueni Bordatchev

As seen in ICALEO 2017 and LIA TODAY

 

Nature is a bountiful source of inspiration to advance innovative surface functionalities, processes, and technologies for engineering materials. For example, the super-hydrophobic surface characteristic of the lotus leaf can be recreated by mimicking the microstructure and surface energy on stainless steels. This super-hydrophobic behavior, which causes water to roll off the lotus leaf while collecting dust particles, enables the self-cleaning of the leaf surface and is primarily due to the hierarchical conical structures, as well as the wax layer present on the leaf surface. A good understanding of the surface topography of the microstructures, water droplet contact angle, and surface chemical composition provides the important clues necessary for the creation of artificial super-hydrophobic or superhydrophilic surfaces and using state-of-the-art ultrafast laser ablation treatment.

Figure 1

Controlling the wettability of a material surface for superhydrophobic or superhydrophilic performance has been an interesting area where numerous different methods are being pursued. While many coatings and thin-films are able to achieve extremely high or low wettability, their endurance life, chemical compatibility, and large area scalability make them less attractive for manufacturing environments. Meanwhile, ultrafast pulsed lasers with several megahertz pulse repetition rates can tune the wettability of a surface without changing its chemical composition and offers higher endurance lives. This is accomplished by instant vaporization (laser ablation) of the material in specific micro-scale patterns thus creating structures that changes the way the surface topography interacts with water.

A superhydrophobic surface is characterized by its ability to repel water using structures that are akin to a bed of nails allowing the water droplet to rest only on the peaks using surface tension and therefore repel from the surface (see Fig.1). Contrarily, a superhydrophilic surface is characterized by its ability to attract and spread the water so features a series of channels that trap water and wick it away using micro-capillary forces. Such surface functionalization techniques have been developed at Canada’s National Research Council for stainless steel (304 SS) and Silicon Carbide (SiC) surfaces respectively to demonstrate the effectiveness of laser texturing technology for wettability control of common engineering surfaces. Fig.2 depicts superhydrophobic performance of a bouncing water droplet at ~5° tilt on 3×3 cm2 textured area.

Experimentally, a 10 W picosecond pulsed laser operating at 1 MHz frequency was focused to a tiny spot of 25 µm diameter. The samples were mounted on a CNC motion system equipped with argon gas protective environment. The optimization of laser structuring process included varying each of the laser parameters, e.g. power, frequency, feed rate, grid pitch, etc. and evaluating the water droplet contact angle using the standard drop-shape analysis method. For the 304 SS superhydrophobic surface, a laser beam fluence of 2.61 J/cm2 was used to promote narrower, shallower features by material redistribution rather than complete vaporization, while the SiC superhydrophilic surface was realized using a much higher fluence of 10.7 J/cm2 to create thicker and deeper channels for the water to impregnate. Both surfaces were machined using the five-axis CNC micromachining system to texture grid patterns, ensuring an even distribution of micro-structures.

Figure 2

 

The superhydrophobicity of 304 SS surface was highly dependent on post-processing conditions in order to tune the wettability. Specifically, the chemical nature of the surface was reactive for 14 days after laser processing due to high-power interaction with the material which excites the chemical state. The samples were thus stored in different environments and exhibited vastly different contact angles. Most notably, the sample which was submerged in deionized water showed hydrophilic tendencies while the sample kept in extremely dry (<8% relative humidity) air was highly superhydrophobic with a contact angle of 152º. Following this two week period, the sample attained stable chemical equilibrium and the wettability was unchanged regardless of environment.

Figure 3

The superhydrophilic SiC surface on the other hand was not as reactive and therefore showed a contact angle of 0º immediately after processing. As aforementioned this sample was intended to have wider and deeper channels to hold and wick the water away from the contact point. The micro-capillary forces that are responsible for spreading the water across the surface were strong enough even to counter gravity; Figure 3 shown below depicts a time-lapse of a 3×3 cm2 textured area placed vertically with the bottom edge in water. Within a 10-second span, the entire surface was wet by the micro-capillary forces pulling water vertically against the force of gravity.

The potential for laser texturing technologies spans many applications in manufacturing industries. Superhydrophobic surfaces have been proposed as a method to mitigate many fluid problems; by decreasing the interaction between a pipe wall and the fluid, the drag experienced by the fluid has been shown to decrease significantly in both laminar and turbulent flows. Thus far, only superhydrophobic coatings and thin-films have been tested for this application however they remain plagued by rapid wear and very short lifetimes. The robustness of the laser texturing process to achieve superhydrophobicity therefore presents exciting new opportunities. As well as water repellency of superhydrophobic surfaces, longer freezing times of water droplets and lower adhesion strength of ice to the surface are characteristics of these high contact angle surfaces and thus present an iceophobic surface property. This enables applications for machinery that operate in colder climates such as wind turbines and airplane wings and engines.

Applications for superhydrophilic surfaces are commonly based on the micro-capillary forces demonstrated as the rapid dispersion creates a thin film of water on the surface. This thin film allows for an increased rate of evaporation from the surface opening doors for anti-fogging applications or greatly increased rates of heat transfer. Other applications manipulate the thickness of the film formed which can provide antireflection ability for surfaces such as solar cells. Superhydrophilic textured surfaces also exhibit increased adhesion strength with the liquid due to the impregnation of the liquid into the surface, therefore providing applications for improvement in bonding strength of joints between different material surfaces.

The wettability control functionalization on engineering surfaces opens the door for new applications with both superhydrophobic and superhydrophilic surfaces. The robust nature of laser surface texturing technologies in combination with chemical compatibility and industrial scalability makes this method unique and most promising to deploy a wide range of functions in manufacturing products. While this technology has already provided solutions to several significant industrial tasks, many more applications are currently being explored at NRC.

 

More details on this topic can be found on YouTube: Combined Wettability Control (https://youtu.be/7IW2aC_rkjw), Super-hydrophobic Bouncing (https://youtu.be/b1vXDuvf3aQ), Super-hydrophilic Ceramic (https://youtu.be/9ZCcW4cOccw), along with other presentations on NRC’s micro/nano-machining capabilities. Further details on these studies can be found in: Superhydrophobic and superhydrophilic functionalized surfaces by picosecond laser texturing. Journal of Laser Applications 30, 032505 (2018); https://doi.org/10.2351/1.5040641

Laser direct writing: An effective approach to flexible hybrid electronics

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As featured in the LIA TODAY and ICALEO 2017

By Jinguang Cai and Akira Watanabe

The development of miniaturized portable and wearable electronic devices has attracted worldwide research attention, due to their increasing integration with human daily life. Different from traditional electronic devices on “hard” boards, such devices should be soft and flexible. A concept called “flexible hybrid electronics”, which is a hybrid of soft and hard parts, has been proposed to address the fabrication issue of flexible devices.

Fig. 1. A schematic image of a simple flexible hybrid electronic device.

The concept of a flexible hybrid electronic device is illustrated in Fig.1, in which “soft parts”, such as electronic interconnection, energy harvesting and energy storage components, antennae, and even displays, sensors, and communication interfaces, can be prepared via printing methods. “hard parts”, such as processor and memory, can be provided by the commercial small-size silicon components, which are small enough, and can be integrated into the device without influencing the flexibility. However, printing methods have some issues such as limitation of materials, high cost of inks, and complicated post-processing.

Laser direct writing is a non-contact, fast single-step fabrication technique without requirements for masks, post-processing, and complex clean environments. Meanwhile, various kinds of lasers have been rapidly developed with relatively low costs and broad available wavelengths and powers, and widely used in industry for materials welding, cutting, and polishing. Besides, laser processing can be focused into a micrometer-sized or submicrometer-sized area to realize on-demand fabrication of functional micro-patterns, showing the potential to be integrated into current product lines for commercial use. Laser direct writing has been demonstrated to prepare energy harvesting and energy storage components, electronic circuits, sensors, communication interface, and antennae.

Energy harvesting and storage components

Generally, a flexible device employs a thin film battery as the energy storage and supply unit, in combination with a flexible energy harvesting device such as a polymer solar cell. Recently, as a new type of energy storage device, micro-supercapacitors (MSCs), has been developed and recognized as potential power supply units for on-chip micro-devices, because they possess not only the advantages of supercapacitors such as high power density, excellent cycling stability, pollution-free operation, maintenance-free feature, and flexibility, but also simplified packaging processes and compatibility with integrated circuits. Among various materials for supercapacitors, carbon materials possess the properties which can satisfy the requirements of MSCs in the flexible devices, such as high specific surface area, high electrical conductivity, high electrochemical stability, and high mechanical tolerance.

Fig. 2. A carbon micro-supercapacitor prepared by laser direct writing.

Laser-induced carbonization of polymers such as polyimide has been demonstrated to prepare flexible all-solid-state carbon-based MSCs with high performance by laser direct writing on a polyimide (PI) film in air (Fig. 2). In order to suppress the oxidation process and thus, increase the conductivity of the laser-induced carbon structures, the laser direct writing was conducted on PI films in an inert gas such as Ar, resulting in carbon MSCs with improved volumetric energy density and power density. Furthermore, high-conductive Au nanoparticles can be incorporated to construct double-layer carbon/Au composite electrodes with improved conductivity by two-step laser direct writing.

It will be beneficial for practical use if energy harvest and storage units can be integrated into the same device. As a demonstration, TiO2 nanoparticles were deposited on one side of the laser-written interdigitated carbon electrodes by an electrophoretic method, forming carbon/TiO2 composite MSC with photo-rechargeable capability under UV irradiation due to the photovoltaic property of TiO2 nanoparticles. Although the charging voltage is not high enough for practical use currently, it is expected that such a strategy can be developed for practical use by optimizing the photo-absorption materials and the combination.

 

Fig. 3. Schematic images of an integrated photodetector (top) and a rGO/GO/rGO humidity sensor (bottom) prepared by laser direct writing.

 

Photodetectors and humidity sensors

Sensors are one of the most important interfaces with users in IoT (Internet of Things) technology, thence it is very important to develop various types of sensors for flexible devices. Laser direct writing can play an important role in preparing sensors directly on the flexible substrate with high performance and stability. For example, a photodetector for UV light can be fabricated by laser direct writing and deposition of ZnO nanoparticles, and can be integrated into the PI film with a carbon MSC fabricated by laser-induced carbonization, forming an integrated photodetector for practical use (Fig. 3, Top). Besides, a humidity sensor based on an interdigitated reduced graphene oxide (rGO)/graphene oxide (GO)/rGO structure prepared by laser direct writing was also demonstrated with high and fast response, flexibility, and long-term stability, showing the potential to be used in flexible devices (Fig. 3, Bottom).

Fig. 4. Schematic images of an integrated wireless charging and storage device (top) and an NFC tag (bottom) prepared by laser direct writing in combination with electroless Ni plating.

Circuits, communication interface, and antenna

High-conductive metallic circuits with mechanical stability are very important in flexible devices as basic structures, and their preparation should be facile, cost-effective, and easily integrated with other electronic components. Laser direct writing has been demonstrated to pattern metallic Pd on PI films, which can act as catalysts in the electroless Ni plating process, producing high-conductive carbon/Ni composite structures. The carbon/Ni structures exhibited a certain flexibility and excellent anti-scratch performance due to the intimate deposition of Ni layer on carbon surfaces. Such carbon/Ni structures can be used as conductive circuits to construct practical devices.  For example, a wireless charging and storage device can be fabricated by integrating an outer rectangle carbon/Ni composite coil for harvesting electromagnetic waves and an inner carbon MSC for energy storage, which can be fast charged by a commercial wireless charger (Fig. 4, Top). In addition, a near-field communication (NFC) antenna was prepared using a carbon/Ni composite coil, and acted as a communication interface with an NFC smartphone for harvesting signals, and an ultra-small commercial IC chip was integrated for data storage (Fig. 4, Bottom). The integrated NFC tag can be used for practical application.

Summary & outlook

While laser direct writing has been demonstrated to be an effective approach to preparing most of the components in flexible devices, such as carbon MSCs as energy storage unit, carbon/TiO2 MSCs for energy harvesting and storage unit, photodetectors and humidity sensors, high-conductive carbon/Ni structures for electronic circuits, and even integrated wireless devices, many efforts are still required to promote the laser direct writing technique applied in the development of flexible hybrid electronics for practical applications in IoT in the future.

 

Jinguang Cai, Institute of Materials, China Academy of Engineering Physics, Jiangyou 621908, Sichuan, P. R. China

Akira Watanabe, Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

Inventors Synthesize Graphene with Lasers

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As featured in the LIA TODAY

By Liliana Caldero

Graphene – it’s the two-dimensional (2D) allotrope of carbon atoms that ignited the imaginations of researchers across the globe. Heralded as a ‘miracle material’, its potential seemed limitless and it was predicted to usher in the next generation of technology. Flexible, stronger than steel, transparent, lightweight, and an amazing conductor of heat and electricity, it was going to revolutionize everything from household electronics to biomedical nanotechnology.

 

THE PROBLEM

Yet, nearly eight years after Dr. Andre Geim and Dr. Konstantin Novoselov earned the Nobel Prize in Physics for first isolating graphene and identifying its properties, graphene has encountered barriers to moving out of the lab and into the marketplace. According to Prof. Dr. Aravinda Kar of the University of Central Florida’s Center for Research and Education in Optics and Lasers (CREOL), one of the most prominent barriers has been finding scalable manufacturing processes that can produce graphene of a quality and quantity ready for consumers and businesses.

Graphene is notoriously difficult to synthesize in large quantities at a consistent quality. Early methods of isolating graphene involved a slow and tedious mechanical exfoliation technique; the researchers would extract a thin layer of graphite from a graphite crystal using regular adhesive tape, continually reducing the graphite sample by sticking the tape together and pulling it apart until only a small, 2D section of carbon atoms with a honeycomb lattice remained. Graphene’s unique characteristics are only present when it is one, two, or three layers of atoms thick – any thicker and it becomes graphite, losing all of the exceptional properties of graphene. The tape exfoliation method, although useful for the lab, was not going to translate very well to an industrialized process.

 

SOLVENT-AIDED EXFOLIATION AND CVD

Two of the more promising and potentially scalable methods of producing graphene are solvent-aided exfoliation and chemical vapor deposition (CVD). In solvent-aided exfoliation, sonication is used to exfoliate graphene crystals which are then further separated in a solvent and later gathered into graphene monolayers.  Scientists at the National University of Singapore have identified a flocculation method that reduces the amount of solvent needed for their exfoliation process, which could yield graphene using far less solvent than was previously needed. Another method experiencing innovation is CVD, which uses thermal chemical reactions to ‘grow’ graphene on substrates of specific materials, typically copper or silicon. Recently, engineers at MIT developed a CVD process for producing graphene filtration membrane sheets at 5 cm per minute. One of the biggest issues with traditional CVD and exfoliation methods is the need to transfer graphene from its fabrication platform to a substrate. Lasers are going to change that.

 

THE MISSING PIECE – LASERS

Lasers may provide yet another avenue to the elusive mass production of graphene, with an eye toward innovating the semiconductor industry. In 2003, Kar, along with Dr. Islam Salama and Dr. Nathaniel Quick, realized that laser direct writing could be used to fabricate carbon-rich nanoribbons on a silicon carbide (SiC) wafer in a nitrogen rich environment. Although these ribbons were too thick to be considered graphene, Kar believed that with a few changes, this process could be reworked to synthesize graphene in situ on a large scale, very quickly. In 2013, Kar and Quick were issued a patent for a Laser Chemical Vapor Deposition (LCVD) method that could be scaled for mass production.

Their method involved a few simple components: a frequency doubled Nd:YAG (green) laser of 532 nm wavelength, methane (CH4) gas, a silicon substrate, and a vacuum chamber.

The 532 nm wavelength corresponds to a photon of energy 2.33 eV, so the energy of two photons is 4.66 eV, just within the range of the C-H bond energy (4.3-4.85 eV) in CH4. Focusing the laser beam to a high intensity can induce two-photon absorption at the focal plane, causing the decomposition of CH4 to release the hydrogen atoms and deposit carbon atoms only on the substrate. The laser heating of the silicon substrate is just low enough to avoid melting the silicon, while providing sufficient thermal and electromagnetic energies to assist the carbon-carbon bonds rearrange into graphene’s trademark hexagonal pattern.

An experimental set-up for multiphoton photolytic laser chemical vapor deposition (LCVD) of graphene from methane precursor. Image courtesy of Dr. Kar and Dr. Quick.

LASER DIRECT WRITING OF GRAPHENE

Kar believes this process could be adapted to add graphene directly onto any substrate. Using laser direct writing, a company could easily draw graphene circuits onto a board. For companies using a hybrid approach, the graphene could be deposited at precise points as interconnects. “You would have all the CAD/CAM capability you could want,” says Quick. Currently, green lasers are available at high output powers, 100 W in continuous wave mode from most large laser manufacturers, so adding this additional step to the manufacturing pipeline for semiconductors would be easy and inexpensive compared to other methods.

At 1.9 cm per second, or 45 inches per minute, this method of graphene production is fast and efficient. This LCVD method offers control over the number of graphene layers, whether one, two, or three are required.  This process also removes the need to manually place graphene onto its intended location, as it is synthesized precisely where it should be. It’s also worth mentioning that this process is conducive to minimal environmental impact, as the unreacted methane and hydrogen byproducts can be captured to be recycled and reused.

 

A LOOK AT THE FUTURE

Picture this: a template is placed over a substrate and a line-shaped laser beam sweeps over it briefly or a beam of large cross-sectional area illuminates the entire template in one shot; when the template is removed, an intricate graphene design has been printed onto a circuit board. That is the future that Kar says is possible, with the right equipment. He suggests that we need manufacturers to develop lasers producing line-shaped beams or large area beams with spatially uniform intensity profile to realize this vision cost-effectively. He emphasizes that a true line-shaped beam produced by a slab laser system or an array of optical fiber laser would be necessary, as shaping the beam synthetically by changing the shape of an aperture would result in too much lost energy. With this technology, graphene could easily be printed onto circuit boards immediately, only where it’s needed, saving in material costs and time.

Nearly 14 years after the excitement first began, researchers are still exploring the potential uses of graphene; from applications in microsupercapacitors to Organic LEDs in flexible displays to ultra-sensitive optical sensors, and even lightweight body armor, the possibilities are still as exciting as ever.

 

Acknowledgements

Prof. Dr. Aravinda Kar, University of Central Florida, CREOL

Dr. Nathaniel Quick, Executive Director of LIA

 

LEARN MORE

Laser Formation of Graphene: United States Patent 8617669. (N. Quick, A. Kar)
http://www.freepatentsonline.com/8617669.html

NUS-led research team develops cost effective technique for mass production of high-quality graphene – http://news.nus.edu.sg/press-releases/mass-production-graphene-slurry

MIT researchers develop scalable manufacturing process for graphene sheets – https://newatlas.com/mit-manufacturing-graphene-filtration-membranes/54274/

High Density Through Glass Vias for Advanced Chip Packaging

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By Ralph Delmdahl, Rainer Pätzel, Rolf Senczuk and Jan Brune

Glass is a promising material from which advanced interposers for high density electrical interconnects for 2.5D and 3D chip packaging can be produced. The supply of ultra-thin glass wafers with thicknesses of 100 µm and below shows attractive cost and superior high frequency performance relative to polished thin wafers made of silicon. As thin glass is extremely brittle, micromachining to create through glass vias is particularly challenging and laser processing using deep UV excimer lasers at a wavelength of 193 nm and ultra-short pulse laser provide a viable solution. Continue reading →

Femtosecond Laser Micromachining of Fused Silica for Microfluidic and Micro-Optical Applications

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By: Ya Cheng, Fei He, Yang Liao, Lingling Qiao, Zhizhan Xu, Koji Sugioka and Katsumi Midorikawa

State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, China
Laser Technology Laboratory, RIKEN – Advanced Science Institute, Japan

Nowadays, microfluidic systems for controlling and manipulating tiny volumes of liquids with high precision have attracted significant attention due to their capability of downsizing both chemistry and biology. In addition, it is often desirable to incorporate micro-optical structures into the microfluidic chips, which leads to not only compact chemical and biological sensors but also tunable and reconfigurable photonic devices. For both microfluidic and micro-optical applications, fused silica can be an ideal substrate material due to its excellent physical and chemical properties, such as chemical inertness, low thermal expansion coefficient, low autofluorescence, exceptional transmittance over a wide spectral range, and so on. On the other hand, fabrication of three-dimensional (3D) microstructures with fused silica, including embedded microfluidic channels and microspherical optical lenses, has long been a challenge because traditional approaches based on photolithography inherently produce planar structures. Here, we show that 3D micromachining of fused silica for both microfluidic and micro-optical applications can be achieved using femtosecond laser direct writing followed by a wet etching in hydrofluoric (HF) acid. In this process, the internal areas modified by the femtosecond laser irradiation will gain a significantly higher etch rate than those unmodified areas, so that hollow structures embedded in fused silica can be produced by preferentially removing the materials in the laser-scanned areas.

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