J. Micro Nano-Manuf. 2014;2(3):030201-030201-1. doi:10.1115/1.4027803.

Micro- and nano-fabrication processes are increasingly important in modern technology and economy. Basic fabrication methods include additive, subtractive and deformation-based processes. In recent years, novel fabrication processes such as laser micro/nano machining, 3D printing of miniature features, etc., have undergone significant growth and have reduced the manufacturing cost and enabled new designs for emerging markets such as smart phones, photovoltaics (PVs), and advanced batteries.

Topics: Manufacturing
Commentary by Dr. Valentin Fuster

Research Papers

J. Micro Nano-Manuf. 2014;2(3):031001-031001-9. doi:10.1115/1.4027738.

The ultrashort pulsed laser ablation process is a well-established micromachining process and has been at the center of manufacturing research in the past decade. However, it has its own limitations, primarily due to the involvement of various material-specific laser and machining process parameters. The laser-induced plasma micromachining (LIP-MM) is a novel tool-less and multimaterial selective material removal type of micromachining process. In a manner similar to ultrashort pulsed laser ablation, it also removes material through an ultrashort pulsed laser beam. However, instead of direct laser–matter interaction, it uses the laser beam to generate plasma within a transparent dielectric media that facilitates material removal through plasma–matter interaction and thus circumvents some of the limitations associated with the ultrashort pulsed laser ablation process. This paper presents an experimental investigation on the comparative assessment of the capabilities of the two processes in the machining of microchannels in stainless steel. For this purpose, microchannels were machined by the two processes at similar pulse energy levels and feed-rate values. The comparative assessment was based on the geometric characteristics, material removal rate (MRR), heat-affected zone and shock-affected zone (HAZ, SAZ), and the range of machinable materials.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2014;2(3):031002-031002-8. doi:10.1115/1.4027547.

Rotational temperature profiles of H2 in a microwave plasma chemical vapor deposition (MPCVD) reactor were measured via coherent anti-Stokes Raman scattering (CARS) spectroscopy. The temperature was found to increase with reactor pressure, plasma generator power, and distance from the deposition surface. At 10 Torr, the measured temperature range was approximately 700–1200 K while at 30 Torr it was 1200–2000 K under the conditions studied. The introduction of CH4 and N2 to the plasma increased the rotational temperature consistently. These findings will aid in understanding the function of the chemical composition and reactions in the plasma environment of these reactors which, to date, remains obscure.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2014;2(3):031003-031003-10. doi:10.1115/1.4027589.

Plasmonic lithography may become a mainstream nanofabrication technique in the future. Experimental results show that feature size with 22 nm resolution can be achieved by plasmonic lithography. In the experiment, a plasmonic lens (PL) is used to focus the laser energy with resolution much higher than the diffraction limit and features are created in the thermally sensitive phase-change material (PCM) layer. The energy transport mechanisms are still not fully understood in the lithography process. In order to predict the lithography resolution and explore the energy transport mechanisms involved in the process, customized electromagnetic wave (EMW) and heat transfer (HT) models were developed in comsol. Parametric studies on both operating parameters and material properties were performed to optimize the lithography process. The parametric studies show that the lithography process can be improved by either reducing the thickness of the phase-change material layer or using a material with smaller real refractive index for that layer.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2014;2(3):031004-031004-8. doi:10.1115/1.4027740.

A reverse oscillatory flow (ROF) mixing system is discussed having a reaction channel 460 μm high by 152 mm wide for high flow rate processing of nanoparticle (NP) chemistries. The ROF system is demonstrated to produce CdS nanoparticles at a production rate of 115.7 g/h with a coefficient of variation (CV) for particle size down to 19%. These production rates are substantially higher than those achieved using other microchannel mixers while maintaining comparable size distributions. Advantages of the ROF approach include the use of larger microchannels which make the reactor easier to fabricate and less vulnerable to clogging.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2014;2(3):031005-031005-7. doi:10.1115/1.4027811.

Presently surface microtexturing has found many promising applications in the fields of tribology, biomedical engineering, metal cutting, and other functional or topographical surfaces. Most of these applications are material-specific, which necessitates the need for a texturing and machining process that surpasses the limitations posed by a certain class of materials that are difficult to process by laser ablation, owing to their optical or other surface or bulk characteristics. Laser induced plasma micromachining (LIPMM) has emerged as a promising alternative to direct laser ablation for micromachining and microtexturing, which offers superior machining characteristics while preserving the resolution, accuracy and tool-less nature of laser ablation. This study is aimed at understanding the capability of LIPMM process to address some of the issues faced by pulsed laser ablation in material processing. This paper experimentally demonstrates machining of optically transmissive, reflective, and rough surface materials using LIPMM. Apart from this, the study includes machining of conventional metals (nickel and titanium) and polymer (polyimide), to demonstrate higher obtainable depth and reduced heat-affected distortion around microfeatures machined by LIPMM, as compared to laser ablation.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2014;2(3):031006-031006-9. doi:10.1115/1.4027778.

Most end stage renal disease patients receive kidney hemodialysis three to four times per week at central medical facilities. At-home kidney dialysis increases the convenience and frequency of hemodialysis treatments which has been shown to produce better patient outcomes. One limiting factor in realizing home hemodialysis treatments is the cost of the hemodialyzer. Microchannel hemodialyzers produced using compression sealing techniques show promise for reducing the size and cost of hemodialyzers. Challenges include the use of a 25 μm thick elastoviscoplastic (EVP) mass transfer membrane for gasketing. This paper provides a framework for understanding the hermeticity of these compression seals. The mechanical properties of a Gambro AN69ST membrane are determined and used to establish limits on the dimensional tolerances of the polycarbonate (PC) laminae containing sealing bosses used to seal the hemodialyzer. The resulting methods are applied to the fabrication of a hemodialysis device showing constraints on the scaling of this method to larger device sizes. The resulting hemodialysis device is used to perform urea mass transfer experiments without leakage.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2014;2(3):031007-031007-7. doi:10.1115/1.4027733.

In this paper, precise P3 scribing of thin-film solar cells (AZO/CIGS/Mo/Glass) via a picosecond laser is investigated. A parametric study is carried out for P3 scribing to study the effects of laser fluence and overlap ratio on ablation depth and slot quality, supported by the numerical prediction using a two-temperature model. The optimum scribing conditions are determined, and the potential processing speed is increased. Laser induced periodic surface structures are also presented after the scribing process, which can potentially enhance the absorption of the cell surface and consequently increase the cell efficiency.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Micro Nano-Manuf. 2014;2(3):034501-034501-5. doi:10.1115/1.4027737.

Two-photon polymerization (2PP) is a powerful technique in fabricating three-dimensional subdiffraction-limited structures. In this paper, 2PP was applied to generate woodpile structures, one kind of photonic crystal, using SZ2080, which is widely used in 2PP due to its negligible shrinkage. First, the relationship between scanning speed, laser power, and resolution was determined through fabricating free-hanging lines by theoretical and experimental study. Based on this relationship, woodpile structures with different period distances were fabricated with high uniformity as shown by scanning electron microscopy (SEM) images. Then optical properties of woodpile structures were investigated using Fourier transform infrared spectroscopy (FTIR) and a quantitative empirical relationship between period distance and band gaps was established. The empirical relationship can be applied to design woodpile photonic crystals for the optical sensors and filters.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2014;2(3):034502-034502-6. doi:10.1115/1.4027810.

Recent advances in manufacturing techniques have opened up new interest in rapid prototyping at the microscale. Traditionally microscale devices are fabricated using photolithography, however this process can be time consuming, challenging, and expensive. This paper focuses on three promising rapid prototyping techniques: laser ablation, micromilling, and 3D printing. Emphasis is given to rapid prototyping tools that are commercially available to the research community rather those only used in manufacturing research. Due to the interest in rapid prototyping within the microfluidics community a test part was designed with microfluidic features. This test part was then manufactured using the three different rapid prototyping methods. Accuracy of the features and surface roughness were measured using a surface profilometer, scanning electron microscope (SEM), and optical microscope. Micromilling was found to produce the most accurate features and best surface finish down to ∼100 μm, however it did not achieve the small feature sizes produced by laser ablation. The 3D printed part, though easily manufactured, did not achieve feature sizes small enough for most microfluidic applications. Laser ablation created somewhat rough and erratic channels, however the process was faster and achieved features smaller than either of the other two methods.

Commentary by Dr. Valentin Fuster

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