Driven by functionality and purity demand for applications of inorganic nanoparticle colloids in optics, biology, and energy, their surface chemistry has become a topic of intensive research interest. Consequently, ligand-free colloids are ideal reference materials for evaluating the effects of surface adsorbates from the initial state for application-oriented nanointegration purposes. After two decades of development, laser synthesis and processing of colloids (LSPC) has emerged as a convenient and scalable technique for the synthesis of ligand-free nanomaterials in sealed environments. In addition to the high-purity surface of LSPC-generated nanoparticles, other strengths of LSPC include its high throughput, convenience for preparing alloys or series of doped nanomaterials, and its continuous operation mode, suitable for downstream processing. Unscreened surface charge of LSPC-synthesized colloids is the key to achieving colloidal stability and high affinity to biomolecules as well as support materials,...

Laser Synthesis and Processing of Colloids: Fundamentals and Applications

The mechanisms of short pulse laser interactions with a metal target are investigated in simulations performed with a model combining the molecular dynamics method with a continuum description of laser excitation, electron−phonon equilibration, and electron heat conduction. Three regimes of material response to laser irradiation are identified in simulations performed with a 1 ps laser pulse, which corresponds to the condition of stress confinement: melting and resolidification of a surface region of the target, photomechanical spallation of a single or multiple layers or droplets, and an explosive disintegration of an overheated surface layer (phase explosion). The processes of laser melting, spallation, and phase explosion are taking place on the same time scale and are closely intertwined with each other. The transition to the spallation regime results in a reduction of the melting zone and a sharp drop in the duration of the melting and resolidification cycle. The transition from spallation to phase e...

Atomistic Modeling of Short Pulse Laser Ablation of Metals: Connections between Melting, Spallation, and Phase Explosion†

https://publik.tuwien.ac.at/files/PubDat_198014.pdf

Metal Ablation with Short and Ultrashort Laser Pulses

A brief review is given regarding ultrafast laser micromachining of materials. Some general experimental observations are first provided to show the characteristics of ultrafast laser micromachining. Apart from empirical research, mathematical models also appear to allow for a further and systematic understanding of these phenomena. A few fundamental ultrafast laser micromachining mechanisms are addressed in an attempt to highlight the physics behind the experimental observations and the mathematical models. It is supposed that a vivid view of ultrafast laser micromachining has been presented by linking experimental observations, mathematical models and the behind physics.

A review of ultrafast laser materials micromachining

Femtosecond laser micromachining has emerged in recent years as a new technique for micro/nano structure fabrication because of its applicability to virtually all kinds of materials in an easy one-step process that is scalable. In the past, much research on femtosecond laser micromachining was carried out to understand the complex ablation mechanism, whereas recent works are mostly concerned with the fabrication of surface structures because of their numerous possible applications. The state-of-the-art knowledge on the fabrication of these structures on metals with direct femtosecond laser micromachining is reviewed in this article. The effect of various parameters, such as fluence, number of pulses, laser beam polarization, wavelength, incident angle, scan velocity, number of scans, and environment, on the formation of different structures is discussed in detail wherever possible. Furthermore, a guideline for surface structures optimization is provided. The authors’ experimental work on laser-inscribed regular pattern fabrication is presented to give a complete picture of micromachining processes. Finally, possible applications of laser-machined surface structures in different fields are briefly reviewed.

https://www.mdpi.com/2072-666X/5/4/1219/pdf

Fabrication of Micro/Nano Structures on Metals by Femtosecond Laser Micromachining

The effect of damage accumulation behaviour on ablation thresholds and damage morphology in ultrafast laser micro-machining of common metals in air

This work reports on the structural changes that take place in wafer grade silicon when it is micro-machined with ultra-short laser pulses of 150 fs duration. A Chirped Pulse Amplification (CPA) Ti:Sapphire laser was used, with an operating wavelength centered on 775 nm and a maximum repetition rate of 1 KHz. The laser induced damage was characterized over the fluence range 0.43–14 J cm-2, and for each fluence a progressively increasing number of pulses was used. The analytical tools used to characterize the samples were all based upon electron microscopy. A 30 KeV scanning transmission electron microscope (STEM) imaging technique was developed to observe defects in the crystal lattice and the thermal-mechanical damage in the area surrounding the laser machined region. Mechanical cross sectioning (in conjunction with Scanning Electron Microscope (SEM) surface imaging) was also used to reveal the internal structure, composition, and dimensions of the laser machined structures. Based on this analysis, it will be shown that laser machining of silicon with femtosecond pulses can produce features with minimal thermal damage, although lattice damage created by mechanical stresses and the deposition of ablated material both limit the extent to which this can be achieved, particularly with high aspect ratios. A key feature of the work presented here is the high-resolution STEM images of the laser-machined structures.

STEM (scanning transmission electron microscopy) analysis of femtosecond laser pulse induced damage to bulk silicon

This article studies a reliability mechanism for field-plate-assisted reduced surface field (RESURF) effect 225-V NDMOS devices based on anode hole oxide injection for carriers that are in thermal equilibrium with the surrounding lattice. The injection mechanism is facilitated by a unique combination of layout and application biases that result in electric field vectors pulling low-energy minority hole carriers into oxide traps that overlap the drain potential. The effect of this positive charge trapping along the field oxide is to inhibit the RESURF mechanism, while also weakening the gate oxide where it overlaps the drain that ultimately results in a rupture. To quantify the process, a new accelerated aging technique is described that uses the parasitic n-p-n bipolar parallel of the nMOS channel to significantly increase the number of holes while still maintaining the MOS application voltages needed to enable the mechanism. This provides a cost-effective way to accurately accelerate this reliability mechanism with significantly smaller lead times relative to using higher biases and temperatures. This technique is then used with technical computer-aided design (TCAD) analysis to determine the impact of different manufacturing variables, where process controls are introduced for targeted manufacturing limits that can eliminate this mechanism from occurring.

Cold Carrier Injection Mechanism for Gate Oxide Integrity of High Voltage NDMOS

This article studies the design strategy to build operating and manufacturing robustness into a metal and polysilicon field plate-assisted Reduced Surface Field Effect (RESURF) <inline-formula> <tex-math notation="LaTeX">$\vert 225 \text{V}\vert $ </tex-math></inline-formula> P-type double diffuse metal oxide silicon (PDMOS) device against Breakdown Voltage Walk-In. The article uses technology computer-aided design (TCAD) to calibrate the unique walk-in properties of charge trapping along the thermal oxide interface of the extended drift region with a trap density of <inline-formula> <tex-math notation="LaTeX">$7{e}^{{11}}$ </tex-math></inline-formula> eV<inline-formula> <tex-math notation="LaTeX">$^{-{1}}$ </tex-math></inline-formula>cm<inline-formula> <tex-math notation="LaTeX">$^{-{2}}$ </tex-math></inline-formula>, a trap occupancy breakdown voltage temperature coefficient of 1.9 V/K, and a breakdown voltage walk-in plateau of −133 V determined by the opposing electric field from the fully occupied traps inhibiting the resurf mechanism. Using this calibration, the article goes on to solve the walk-in mechanism by using a junction resurf field plate buried in the silicon lattice to create an electric field from the depleted dopant that balances the effects of the oxide-trapped charges. The design of the buried resurf field plate is optimized with TCAD for a robust <inline-formula> <tex-math notation="LaTeX">$\vert 225 \text{V}\vert $ </tex-math></inline-formula> operating voltage. This robustness is confirmed with measured silicon data.

E. Coyne

Papers

The effect of damage accumulation behaviour on ablation thresholds and damage morphology in ultrafast laser micro-machining of common metals in air

STEM (scanning transmission electron microscopy) analysis of femtosecond laser pulse induced damage to bulk silicon

Cold Carrier Injection Mechanism for Gate Oxide Integrity of High Voltage NDMOS

High Voltage PDMOS Device Design Against Breakdown Voltage Walk-In