Laser nanomachining and nanoprocessing ñ frontiers beyond the optical diffraction limit

In semiconductor and data storage device manufacturing, it is desirable to produce feature sizes less than 30 nm with a high depth-to-width aspect ratio on the target material rapidly at a low cost. However, optical diffraction limits the smallest focused laser beam diameter to around half of the laser wavelength (λ/2). The existing approach to achieving nanoscale fabrication is mainly based on costly extreme ultraviolet (EUV) technology operating within the diffraction limit. In this paper, a new method is shown to achieve materials processing resolution down to 10 nm (λ/80) at an infrared laser wavelength of around 800 nm in the far-field, in air, well beyond the optical diffraction limit. A high-quality longitudinal field with a purity of 94.7% is generated to realise this super-resolution. Both experiments and theoretical modelling have been carried out to verify and understand the findings. The ablation craters induced on polished silicon, copper, and sapphire are compared for different types of light fields. Holes of 10–30 nm in diameter are produced on sapphire with a depth-to-width aspect ratio of over 16 and a zero taper with a single pulse at 100–120 nJ pulse energy. Such high aspect ratio sub-50 nm holes produced with single pulse laser irradiation are rarely seen in laser processing, indicating a new material removal mechanism with the longitudinal field. The working distance (lens to target) is around 170 µm, thus the material processing is in the far field. Tapered nano-holes can also be produced by adjusting the lens to the target distance.

A plume consisting of vapor and ionized particles of the workpiece is usually formed in many laser materials processing. The characteristics of this laser-plasma plume depend on a large number of parameters such as the laser power, spot size, scanning speed, material properties and shielding gas. The temperature, degree of ionization, absorption coefficient and pressure of the plume are calculated for different process parameters. The absorption coefficient of the plume given by the Kramers-Unsold relation equation and the ionization fraction given by the Saha-Eggert equation are used to determine the plume pressure. The quality of materials processing can be assessed by the plume properties, and the plume pressure monitoring can be used as a diagnostic tool for process automation and reproducibility of product quality.A plume consisting of vapor and ionized particles of the workpiece is usually formed in many laser materials processing. The characteristics of this laser-plasma plume depend on a large number of parameters such as the laser power, spot size, scanning speed, material properties and shielding gas. The temperature, degree of ionization, absorption coefficient and pressure of the plume are calculated for different process parameters. The absorption coefficient of the plume given by the Kramers-Unsold relation equation and the ionization fraction given by the Saha-Eggert equation are used to determine the plume pressure. The quality of materials processing can be assessed by the plume properties, and the plume pressure monitoring can be used as a diagnostic tool for process automation and reproducibility of product quality.

Lasers have been in use for material processing for decades as a flexible tool for different processes i.e. welding, cutting, drilling, surface modifications etc. Laser material processing offers a number of advantages over other processing technologies, i.e. high processing speeds, low heat input (minimal disruption to the parent material and coatings), low distortion of the parts, better cosmetic appearance and high flexibility. In the past, lamp pumped Nd: YAG lasers have been widely been used for a number micro and macro applications. Today, however, the parameter range of solid state lasers has increased significantly, especially after the emerging of fiber and disk lasers.The choice of a laser source for a specific application is no longer straightforward and obvious. Laser users are now faced with additional questions of laser beam quality and brightness. The main advantage of high beam quality is that the laser beam can be transported via a small diameter optical fiber, which translates to a small spot size at the workpiece.Laser sources with better beam quality are being made available at increasing power levels. However, choosing the optimum laser for an application may not be simply the selection of the highest beam quality at the required power level. This paper addresses this important issue — how the laser beam quality of solid state lasers affects the laser material processing performances by comparing welding and cutting data for thin and thick section of materials.

The complexity of laser material processing can be greatly reduced using computer-generated phase reflection holographic scanners. These scanners direct and focus the beam of a carbon dioxide laser into a spot on the workpiece and then translate this spot over some general 2-D pattern as the scanner undergoes a simple 1-D motion. Procedures for constructing these scanners are presented, and the first-order aberrations introduced by them are analyzed. The primary aberrations cause the diffracted beam to focus to an astigmatic spot on the work surface. The severity of the astigmatism is proportional to the scan rate, scan angle, and f/number. A technique is presented in which the design of the scanner is adjusted so that the astigmatic image is aligned with the scan direction. The resolution perpendicular to the scan direction is the same as that of a scanner without aberrations of the same f/number. Materials processed using these scanners are presented to show their capabilities for carbon dioxide laser material processing. Power densities on the order of 10(6)/cm(2) can be readily obtained using the proposed technique.

Mode-locked oscillators are fantastic scientific instruments. Today, femtosecond pulses are now in demand for applications outside physics and chemistry laboratories, most notably for precision materials processing and nonlinear optical microscopy in biomedical laboratories and clinics. These applications require short pulses with high peak power and excellent beam quality. However, they also require devices that are cheap, reliable, and – crucially -- flexible. For example, synchronization of pulses with scanning optics is crucial for high efficiency machining and imaging alike. Lasers in materials processing are often required to exhibit pulse-on-demand operation in order to deposit energy only in specific locations. In biology, such behavior would enable region-of-interest monitoring, allowing e.g. ultrahigh resolution monitoring of a neuronal circuit’s dynamics. This kind of behavior is not compatible with the regular pulse train of a mode-locked oscillator. To this end, we seed an ultrafast fiber amplifier with a gain-switched diode. This a robust, integrated device that can be electronically triggered at virtually any repetition rate, or on-demand, yielding partially-coherent, ~10 ps duration pulses. Remarkably, a single Mamyshev regenerator (nonlinear spectral broadening in fiber followed by offset spectral filtering) allows isolation of a coherent component of the diode pulse. After subsequent nonlinear pulse shaping, amplification to the µJ-regime, and linear compression, we obtain transform-limited ~140-fs pulses with 13 MW peak power. As will be discussed, such a source cannot replace all mode-locked oscillators (e.g., for frequency comb applications), but is extremely attractive for the key mainstream applications of ultrashort pulses.

A blue diode laser has a higher absorption rate than a traditional laser, while the maximum power is limited. We report the structure and laser beam profile of a 250W high-power blue laser (445nm) for material processing. The absorption rate of the blue laser system for the steel was 2.75 times that of a single-mode fiber laser system (1070nm). The characteristics of the steel after laser irradiation were determined, validating the potential of this high-power blue laser for material processing, such as heat treatment and cladding. The cost of the developed laser system was lower than that of the existing one. To the best of our knowledge, this is the first blue laser with a power as high as 250W.

Thin films are often encountered in micro systems. They are important in semiconductor devices, in microelectromechanical devices such as sensors and actuators, and in masks. Heat transport through thin films is of vital importance in microtechnology applications. In some electronic applications, the reduction of the device size to microscale has the advantage of enhancing the switching speed of the device. On the other hand, size reduction increases the rate of heat generation which can lead to higher thermal load on the device. Heat transfer at the microscale is also important in the processing of materials with pulsed lasers. The high power of the laser on the surface of a metal film can result in a build-up of temperature on the film surface causing thermal damage. When a significant temperature rise occurs in a solid, deformation or highly elevated stress occurs caused by thermal expansion. Deformation and stress waves are major causes of thermal damage in laser processing of materials. Hence, studying the thermal behavior of thin films (in one or in multilayers) is useful for predicting the performance of a microelectronic device or for the manufacturing of microstructures. Heat transfer at the microscale cannot be properly described by the conventional heat transfer equation. In the classical theory of diffusion, the heat flux vector (q) and the temperature gradient (ΔT) across a volume of material are assumed to occur at the same instant of time. However, if the scale in one direction is at the microscale, the flux and temperature gradient in this direction will occur at different times. This time lag requires a new approach in the study of heat transfer at the microscale. In this chapter we discuss heat transfer models in single or multilayer thin films with time lags.KeywordsHeat TransportThermal WaveAmerican Physical SocietyMetal LatticeElectron Thermal ConductivityThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

The conversion of sunlight into laser light by direct solar pumping is of ever-increasing importance because broadband, temporally constant, sunlight is converted into laser light, which can be a source of narrowband, collimated, rapidly pulsed, radiation with the possibility of obtaining extremely high brightness and intensity. Nonlinear processes, such as harmonic generation, might be used to obtain broad wavelength coverage, including the ultraviolet wavelengths, where the solar flux is very weak. The direct excitation of large lasers by sunlight offers the prospect of a drastic reduction in the cost of coherent optical radiation for high average power materials processing. This renewable laser has a large potential for many applications such as high-temperature materials processing, renewable magnesium-hydrogen energy cycle and so on. We propose here a scalable TEM00 mode solar laser pumping scheme, which is composed of four firststage 1.13 m diameter Fresnel lenses with its respective folding mirrors mounted on a two-axis automatic solar tracker. Concentrated solar power at the four focal spots of these Fresnel lenses are focused individually along a common 3.5 mm diameter, 70 mm length Nd:YAG rod via four pairs of second-stage fused-silica spherical lenses and third-stage 2D-CPCs (Compound Parabolic Concentrator), sitting just above the laser rod which is also double-pass pumped by four V-shaped pumping cavities. Distilled water cools both the rod and the concentrators. 15.4 W TEM00 solar laser power is numerically calculated, corresponding to 6.7 times enhancement in laser beam brightness.

PurposeThe paper aims to provide a review of the use of lasers in industry, with an emphasis on applications in manufacturing and material processing.Design/methodology/approachFollowing an introduction to lasers and their development, this paper considers a selection of key material processing applications.FindingsThis shows that, since their invention in 1960, lasers have emerged to play a vital role in numerous industrial processes. Gas (CO2), solid‐state, excimer, semiconductor and fibre lasers are used in welding, soldering, drilling, cutting, micromachining, marking, engraving, heat treatment, material deposition and adhesive curing.Originality/valueThe paper provides a detailed review of the uses of lasers in industrial material processing and also places these applications in an historical context.

Lasers in materials processing

We analyze the general requirements for an economically viable laser materials-processing application. Laser light is not only expensive relative to other forms of energy but at ∼ $10/kg of product for laser processing costs (corresponding to one 2 eV photon per product molecule) it is expensive relative to most bulk chemicals. We identify four criteria for a successful application that allows efficient utilization of this costly source of energy. In reviewing the status of uranium laser isotope separation (LIS) at the Lawrence Livermore National Laboratories (LLNL), we show how this program satisfies our criteria.

Although material processing with high power lasers has found widespread use in a variety of industries such as the automotive industry, electrical and electronics industries, aerospace industry etc., civil engineering construction is one field that has lagged behind in the use of lasers for material processing. This is in spite of the fact that a large variety of materials including ceramics, metals and plastics are used in very large quantities for civil engineering construction. The main reasons for the delay in the adopting of laser for processing construction material seem to be the high costs involved and the lack of sufficient power for processing heavy and thick materials. However, with the advent of more compact lasers with higher powers, higher efficiencies and lower photon costs, greater interest has been shown in recent years in the possible uses of high power lasers for material processing in the construction industry. The author traces some of the past work carried out both in Japan and abroad on the use of lasers in civil engineering, specially with respect to the processing of inorganic material such as concrete, natural stones, tiles and rocks. Recent developments regarding laser decontamination and laser assisted rock excavation are also introduced.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Book Review : Lasers in Materials Processing: A Bibliography of a Developing Technology Compiled by Alan Gomersall copublished by IFS Publications and Springer-Verlag, 1986

Laser processing of materials has achieved significant successes in pulsed laser deposition, micro- and nanostructuring and surface modification and analysis. However, materials scientists often do not think about the physics of those lasers, which determines their properties and therefore also the way in which these lasers can be employed in laser processing. This chapter discusses the essential theory of laser gain, oscillation and amplification, and provides examples drawn from lasers now frequently used in materials processing. The implications for the design of new lasers and new materials-processing strategies are considered, using the example of a picosecond laser system for polymer processing.

The following collection of papers highlights recent developments in the consolidation processing of powder materials. Like hightemperature oxide superconductors, particulate materials M.G. McKlmpson offer tremendous potential for new advancements in materials technology. Today's powder practitioners can exploit a host of powerful techniquesincluding atomization, solid-state-microblending, nanoscale processing, and chemical synthesis-to produce particulates exhibiting properties that cannot be attained using more conventional bulk processing. As with the oxide superconductors, however, the economic potential of these materials can only be realized by developing cost-effective, environmentally acceptable processing technologies for converting powder materials into useful engineering products. A key element of contemporary materials development is the concept of the materials system, which means that a material's performance, processing, and price are interrelated and must be "balanced" in an economically attractive fashion. This concept is clearly not new; the Bessemer converter became a commercial success in the mid-1800s because it produced a new material (steel) with attractive performance characteristics at a much lower cost than the existing "puddling" process for wrought iron. Nevertheless, in today's competitive environment-with its stringent requirements for performance, reliability, and low cost-such a multidimensional systems orientation is absolutely crucial. The following articles nicely illustrate this interrelationship. The first paper, by A. BoseofParmatek, reviews recent developments in powder-injectionmolding.Itbrieflydescribes the injection molding process and points out its principal advantages and limitations from a commercial perspective. The paper discusses the engineering requirements that are well served by injection molding, including size, wall thickness, and dimensional tolerance. In addition, Bose describes the starting powders required for successful injection molding