High-quality Microstructures Research Articles

Fabrication of Spatially Confined Complex Oxides

Complex materials such as high Tc superconductors, multiferroics, and colossal magnetoresistors have electronic and magnetic properties that arise from the inherent strong electron correlations that reside within them. These materials can also possess electronic phase separation in which regions of vastly different resistive and magnetic behavior can coexist within a single crystal alloy material. By reducing the scale of these materials to length scales at and below the inherent size of the electronic domains, novel behaviors can be exposed. Because of this and the fact that spin-charge-lattice-orbital order parameters each involve correlation lengths, spatially reducing these materials for transport measurements is a critical step in understanding the fundamental physics that drives complex behaviors. These materials also offer great potential to become the next generation of electronic devices (1-3). Thus, the fabrication of low dimensional nano- or micro-structures is extremely important to achieve new functionality. This involves multiple controllable processes from high quality thin film growth to accurate electronic property characterization. Here, we present fabrication protocols of high quality microstructures for complex oxide manganite devices. Detailed descriptions and required equipment of thin film growth, photo-lithography, and wire-bonding are presented.

Fabrication of W-12wt%Cu Composites by Powder Metallurgy Method: Activated Sintering

One of the most challenging areas in engineering of composite materials is the fabrication of high quality microstructures. This issue is complicated for tungsten composites due to its high melting temperature and this leads to limitations in terms of possible processing techniques. This research investigates the fabrication of W-12%wtCu composites based on powder metallurgy techniques. Due to the very large difference between the melting point of tungsten and copper, there is no common sintering temperature range for them. In this work, 0.5%Wt nickel was added as an activator to decrease the sintering temperature of tungsten using an activated sintering effect. The effects of pressure, sintering time and temperature in solid state and liquid phase conditions were also investigated. While solid state hot pressing did not result in appropriate microstructures, the liquid phase hot pressing provided high quality samples with a relative density of 98.0%.

Electrically Driven Quantum Dot Micropillar Light Sources

We report on light sources based on electrically pumped quantum dot (QD) micropillar cavities. The low-mode-volume high-quality microstructures feature pronounced cavity quantum electrodynamics (cQED) effects that are exploited for the realization of efficient single-photon sources and low-threshold microlasers. The compact and electrically driven devices are of special interest for applications in the field of quantum communication. In particular, operated as electrically triggered single-photon sources, the QD micropillars can act as building blocks for quantum key distribution and quantum repeaters. On the other hand, the electrically pumped microlasers with in-plane emission via whispering gallery modes or emission normal to the sample's surface are predestinated for integrated light sources in future photonic networks. The devices are based on doped high-quality factor GaAs/AlAs microcavity structures with InGaAs QDs in the active layer. A lateral injection scheme leaves the upper facet of the micropillars free of any absorbing metal and allows for efficient light output under electrical pumping of low-mode-volume micropillars with diameters between 1 and 20 μm. Due to cQED effects, triggered single-photon emission with high photon extraction efficiency up to 62% and a low multiphoton emission probability (g <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(2)</sup> (0) = 0.16) are realized for moderate-quality (Q ) factor samples. The efficient coupling of spontaneous emission into the lasing mode in high- Q micropillars results in ultralow laser threshold currents of less than 10 μA at cryogenic temperatures. Our paper demonstrates the high potential of electrically driven QD micropillars to act as integrated light sources in future communication systems.

Adaptive aberration compensation for three-dimensional micro-fabrication of photonic crystals in lithium niobate

We present the use of a liquid crystal spatial light modulator to correct for the refractive-index mismatch induced spherical aberration in a high refractive-index lithium niobate crystal when a low repetition rate amplified laser is used for the direct fabrication of three-dimensional micro-structures. By correcting the aberration based on experimentally determined values, we show that the size of written structures decreases dramatically, which allows the fabrication of high quality micro-structures such as three-dimensional photonic crystals. We demonstrate that, through the use of adaptive optics, the fabrication depth and the stopgap strength in the corresponding photonic crystals are increased by a factor of two to three.

Fabrication of micro-mold for glass embossing using focused ion beam, femto-second laser, eximer laser and dicing techniques

Abstract Various nano-/micro-structuring methods, including focused ion beam (FIB), femto-second laser, KrF eximer laser and dicing techniques, were used to fabricate glassy carbon (GC) micro-molds and were characterized in terms of the process rate, the roughness and the shape of machined structure. The combination of femto-second laser ablation with subsequent FIB milling provided a possibility for the rapid fabrication of high quality micro-structures on a wide surface area. Obtained molds were applied to the hot-emboss process of glasses to investigate process conditions needed for the replication of structures with various geometries and dimensions.

Microstructuring of glassy carbon mold for glass embossing – Comparison of focused ion beam, nano/femtosecond-pulsed laser and mechanical machining

Various methods, including focused ion beam (FIB), femto-second laser, KrF eximer laser and dicing techniques, were employed for preparing glassy carbon (GC) micro-molds, and those methods were characterized in terms of the process rate, the roughness and the shape of machined structure. FIB milling using a repetitive pass method produced nano/microstructures with flat channel bottom and nearly vertical sidewalls. Although FIB milling was slowest process, it provided the best quality of machined surface ( R a = 4–30 nm depended on milled depth). Femtosecond-pulsed laser machining also allowed fabricating flat bottom and nearly vertical sidewalls on an area of 1.2 × 1.2 mm 2 at a scanning speed of 20 mm/s, but lead to an increase of the surface roughness ( R a = 80 nm). Femto-second laser in combination with FIB milling provided a possibility for the rapid fabrication of high quality microstructures on wide surface area. The roughness of machined surface decreased to 45 nm by the subsequent FIB milling. Microstructuring with a nanosecond-pulsed KrF eximer laser at an irradiation wavelength of 248 nm with a fluence of 13.2 J/cm 2 also allowed the fast fabrication of master structure for micro-gear, resulted in slanted sidewalls and not ideally flat bottoms. The surface roughness ( R a) of the bottom and the side wall was about 45 and 70 nm, respectively. Dicing technique allowed machining micro-channels with a rectangular and pyramidal cross-section on an area of 15 × 15 mm 2 under the feed speed of 50 mm/min. By reducing the feeding speed from 100 mm/min to 50 mm/min, surface roughness ( R a) of the structure side wall decreased from 150 nm to 70 nm. Achieved glassy carbon molds were then applied to the hot-emboss process of Pyrex and quartz glasses to investigate embossing conditions (emboss temperature, pressure and hold time) needed for the replication of Pyrex and quartz glass structures with various geometries and dimensions in glass plates with thickness of 1 mm. Replication results showed good replication at the nanoscale, resulted in the almost the same dimensions and surface roughness with that of cavities. Thicker plate provided faster filling in the emboss process of glass.

Micro-USAL technique for the manufacture of high quality microstructures in brittle materials

Abstract In the paper, microultrasonic assisted lapping (USAL) is proposed as a flexible alternative to micro-system technologies for the manufacture of high surface quality microstructures in brittle materials. The authors first introduce the background and principles of micro-USAL of microstructures, analyze obstacles in implementing and using this technique, and configure a micro-path-controlled USAL test-rig with the integration of on-line AE (acoustic emission) monitoring system. They then discuss the effectiveness of automatic tool–workpiece contact detection and tool wear compensation using AE-based feedback, of which both are important for a stable micro-USAL. They also identify the influence of the AE's positional dependence which was recognized by the authors for the first time. The AE's positional dependence was identified to be inherently attributed to the ultrasonic generator and, under certain conditions, it caused a misjudgment of AE-based feedback. In the paper, they also demonstrate the highly flexible capability of the new micro-USAL in generating microstructures of various geometrical patterns through a number of machining experiments, as well as its current limitations in achievable concave feature sizes, aspect ratios and surface roughness. Also in the paper, the authors finally discuss, in detail, micro-USALed surface properties, effects of process parameters and lapping media, and the newly proposed chemo-mechanical way to improve the micro-USALed surfaces with the achieved minimum roughness of R a 7 nm.

Optical Breakdown of 6H SiC Induced by Wavelength-Tunable Femtosecond Laser Pulses

The damage mechanisms and micromachining of 6H SiC are studied by using femtosecond laser pulses at wavelengths between near infrared (NIR) and near ultraviolet (NUV) delivered from an optical parametric amplifier (OPA). Our experimental results indicate that high quality microstructures can be fabricated in SiC crystals. On the basis of the dependence of the ablated area and the laser pulse energy, the threshold fluence of SiC is found to increase with the incident laser wavelength in the visible region, while it remains almost constant for the NIR laser. For the NIR laser pulses, both photoionization and impact ionization play important roles in electronic excitation, while for visible lasers, photoionization plays a more important role.

Large-scale hot embossing

The hot embossing process is a flexible molding technique to produce delicate microstructures with high aspect ratios on thin layers. Large-scale hot embossing is one effective way to meet future requirements and produce high-quality microstructures at low costs. For this, however, principal changes of the molding process and molding tool design will be required. In the present paper, constructive solutions for large-scale hot embossing shall be described. Based on a simulation of the hot embossing process, solutions shall be presented that are aimed at reducing shrinkage of the molded parts and demolding forces and, hence, at avoiding damages of microstructures during demolding.

Femtosecond laser ablation of polytetrafluoroethylene (Teflon) in ambient air

Teflon, polytetrafluorethylene (PTFE), is an important material in bioscience and medical application due to its special characteristics (bio-compatible, nonflammable, antiadhesive, and heat resistant). The advantages of ultrashort laser processing of Teflon include a minimal thermal penetration region and low processing temperatures, precision removal of material, and good-quality feature definition. In this paper, laser processing of PTFE in ambient air by a Ti:sapphire femtosecond laser (780 nm, 110 fs) is investigated. It is found that the pulse number on each irradiated surface area must be large enough for a clear edge definition and the ablated depth increases with the pulse number. The air ionization effect at high laser fluences not only degrades the ablated structures quality but also reduces the ablation efficiency. High quality microstructures are demonstrated with controlling laser fluence below a critical fluence to exclude the air ionization effect. The ablated microstructures show strong adhesion property to liquids and clear edges that are suitable for bio-implantation applications. Theoretical calculation is used to analyze the evolution of the ablated width and depth at various laser fluences.

102 Ultrasonic Assisted Lapping for the Manufacture of High Surface Quality Microstructures on Brittle Materials

Ultrasonic assisted lapping (USAL) is proposed as a cost-effective alternative to micro-system based technologies, such as lithography, etching, and electron or ion beam micromachining, for manufacturing high surface quality and complex microstructures. The chosen target materials in this research are silicon, glass and advanced ceramics, which are hard-brittle and are not diamond turnable. First this paper describes the principles of USAL for microstructures, followed by obstacles in implementing and using the technique. Then, a path-controlled USAL system with an integrated on-line monitoring AE (Acoustic emission) system is presented. To evaluate the feasibility of the proposed technique, machining experiments were conducted to fabricate linear, spiral and other 2-D arbitrary patterns using a 50μm carbide tool. Further experimental results focus on the achievable surface roughness and observed tool wear as well as on improving the surface roughness by reducing the material removal unit.

Laser ablation of metals and ceramics in picosecond–nanosecond pulsewidth in the presence of different ambient atmospheres

Ablation tests of AlN, Si3N4, SiC, Al2O3 ceramics, steel and aluminum have been carried out in vacuum, air and argon atmospheres using UV (270 nm), visible (539 nm) and IR (1078 nm) picosecond (100÷150 ps) and nanosecond (6÷9 ns) laser pulses. Ablation rate dependencies have been measured in the range of laser energy densities varied from (2÷5)×101 J/cm2 to (5÷10)×103 J/cm2. Peculiarities of laser ablation processes at different wavelengths, pulsewidths and ambient gases are discussed. In particular, the efficiencies of laser ablation in picosecond and nanosecond regions are compared. The scanning electron microscope (SEM) pictures of high quality microstructures, deep and narrow cuts and holes produced in ceramics with typical size of tens microns and aspect ratio as high as 20, are demonstrated.

Laser-Assisted Dry Etching Ablation for Microstructuring of III-V Semiconductors

ABSTRACTLaser-assisted dry etching ablation (LADEA) has been reviewed with an emphasis on its applicability for the microstructuring of III-V semiconductor compounds. The method is based on the application of an excimer laser ( λ= 308 nm) for pulsed heating of a wafer which is placed in a stream of Cl2/He gas. Both the products of chemical reaction and the depth to which a laser-induced reaction takes place depend on laser fluence. This makes possible the ablation of a well defined volume of the material. Little or no structural damage to the surface is observed because ablation is carried out with laser fluences below those required to melt the matrix material. The laser fluence dependence of the etch rate indicates that the process is primarily temperature driven with a characteristic energy for desorption. We have investigated LADEA as a method for in-situ processing of III-V semiconductors and the fabrication of nanostructures. An atomic force microscopy study has shown that atomically smooth surfaces can be obtained if the etch rate is near 1/2 atomic layer per laser pulse. The lateral resolution of LADEA has been found to be at least 20 nm. This, as well as the results of in-situ photoluminescence and Auger electron spectroscopy measurements, indicate that LADEA can be used for the direct (photoresist-free) fabrication of high quality microstructures and, ultimately, for the nanostructuring of III-V semiconductor compounds.

Micromachining of ceramics with excimer laser radiation

A XeCl excimer laser ( λ = 308 nm) was used for investigating the possibilities of micromachining ceramic materials. The influence of the optical setup and laser parameters on the processing quality has been determined, as well as the behaviour of various ceramic materials during the machining process. The machining results and the structure quality are discussed concerning the shape of the holes, break-outs in the structure and the homogeneity of the hole depth and size. Optical microscopy and SEM investigations served as the main tools for the characterization. Strategies for large area micromachining were investigated to achieve a very large number of high quality microstructures. Ein XeCl-Excimerlaser ( λ = 308 nm) wurde zur Mikrostrukturierung keramischer Werkstoffe eingesetzt. Schwerpunkte der Untersuchungen waren der Einfluß des optischen Aufbaus und der verschiedenen Laserparameter auf die Bearbeitungsqualität und die durch den Bearbeitungsprozeß verursachten Werkstoffveränderungen verschiedener keramischer Materialien. Die Form der durch Abtragung erzeugten Strukturen, Ausbruchserscheinungen sowie die Homogenität der Lochtiefen und -größen dienen zur Beschreibung der Bearbeitungsergebnisse. Lichtmikroskopie und REM-Untersuchungen wurden für die Probencharakterisierung herangezogen. Strategien zur großflächigen Bearbeitung wurden mit der Zielsetzung entwickelt, eine sehr große Anzahl von Mikrostrukturen mit hoher Bearbeitungsqualität zu erzeugen. On a utilisé un laser excimer XeCl ( λ = 308 nm) pour étudier les possibilités de microstructurer les matériaux céramiques. On a déterminé l'influence de la construction optique et des paramètres du laser sur la qualité du procédé, ainsi que le comportement des différentes céramiques durant le procédé de fabrication. Les résultats de fabrication et la qualité de la structure sont discutés en rapport avec la forme des trous, les éruptions dans la structure et l'homogénéité de la profondeur et de la taille des trous. Les observations par microscopie optique et MEB sont les outils principaux utilisés pour la caractérisation. On a étudié des stratégies pour travailler de grandes surfaces dans le but d'obtenir un trés grand nombre de microstructure de trés bonne qualité.