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Research Papers

J. Micro Nano-Manuf. 2017;5(3):031001-031001-7. doi:10.1115/1.4036149.

Interface characteristics of Al/Cu microlaminates fabricated by an electrically assisted roll bonding (EARB) process were studied to understand the underlying physical/chemical phenomena that lead to bond strength enhancement when applying electrical current during deformation. Peel tests were conducted for the Al/Cu roll-bonded laminates produced under 0 A, 50 A, and 150 A applied current. After peel tests using a microtensile machine, the fractured surfaces of both the Al and Cu–sides were examined using scanning electron microscopy (SEM) for fractography and SEM-based energy dispersive (EDS) analysis. Results revealed the strong dependence of the fracture path and its morphology on the strength of the bond, which is influenced by various phenomena occurring at the interface during EARB, such as microextrusion through surface microcracks, possible formation of intermetallic components and thermal softening during simultaneous application of strain and high current density.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2017;5(3):031002-031002-10. doi:10.1115/1.4036445.

Two-photon polymerization (TPP) is a laser writing process that enables fabrication of millimeter scale three-dimensional (3D) structures with submicron features. In TPP, writing is achieved via nonlinear two-photon absorption that occurs at high laser intensities. Thus, it is essential to carefully select the incident power to prevent laser damage during polymerization. Currently, the feasible range of laser power is identified by writing small test patterns at varying power levels. Herein, we demonstrate that the results of these tests cannot be generalized, because the damage threshold power depends on the proximity of features and reduces by as much as 47% for overlapping features. We have identified that this reduction occurs primarily due to an increase in the single-photon absorptivity of the resin after curing. We have captured the damage from proximity effects via X-ray 3D computed tomography (CT) images of a nonhomogenous part that has varying feature density. Part damage manifests as internal spherical voids that arise due to boiling of the resist. We have empirically quantified this proximity effect by identifying the damage threshold power at different writing speeds and feature overlap spacings. In addition, we present a first-order analytical model that captures the scaling of this proximity effect. Based on this model and the experiments, we have identified that the proximity effect is more significant at high writing speeds; therefore, it adversely affects the scalability of manufacturing. The scaling laws and the empirical data generated here can be used to select the appropriate TPP writing parameters.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2017;5(3):031003-031003-7. doi:10.1115/1.4036446.

Altering the wetting characteristics of copper will positively impact numerous practical applications. The contact angle (CA) of a water droplet on the polished copper surface is usually between 70 deg and 80 deg. This paper discusses a facile, scalable, tuned bulk micromanufacturing approach for altering the surface topology of copper concomitantly at the micro- and nano-length scales, and thus significantly influence its wetting characteristics. The resultant copper surfaces were found to be robust, nontoxic, and exhibited ultra-omniphilicity to various industrial liquids. This extreme wetting ability akin to a paper towel (CA of zero for multiple liquids) was achieved by tuning the bulk micromanufacturing process to generate connected hierarchical micro- and nano-roughness with nanocavities within the embryos of microcavities. With an adsorbed coating of ester, the same ultra-omniphilic copper surfaces were found to exhibit robust super-hydrophobicity (CA ∼ 152 deg for water).

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2017;5(3):031004-031004-5. doi:10.1115/1.4036828.

The aim of this research is to develop a combined polishing technology for single-crystal silicon carbide (SiC) wafers, which is known to be difficult to process due to its high hardness. This paper proposes a combined polishing method based on converting SiC into a material with a relatively low hardness and then polishing this material using abrasive particles with a higher hardness. An electrochemical technique was tried to reduce the hardness of SiC. The effectiveness of the combined technique is experimentally demonstrated. In addition, the temporal changes of the thickness of SiO2 layer and the relationship between the electrochemical machining current and the thickness of SiO2 layer are shown.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2017;5(3):031005-031005-7. doi:10.1115/1.4036889.

With new fabrication methods for mass production of nanotextured samples, there is an increasing demand for new characterization methods. Conventional microscopes are either too slow and/or too sensitive to vibrations. Scatterometry is a good candidate for in-line measuring in an industrial environment as it is insensitive to vibrations and very fast. However, as common scatterometry techniques are nonimaging, it can be challenging for the operator to find the area of interest on a sample and to detect defects. We have therefore developed the technique imaging scatterometry, in which the user first has to select the area of interest after the data have been acquired. In addition, one is no longer limited to analyze areas equal to the spot size, and areas down to 3 μm × 3 μm can be analyzed. The special method Fourier lens scatterometry is capable of performing measurements on misaligned samples and is therefore suitable in a production line. We demonstrate characterization of one-dimensional and two-dimensional gratings from a single measurement using a Fourier lens scatterometer. In this paper, we present a comparison between spectroscopic scatterometry, the newly developed imaging scatterometry, and some state-of-the-art conventional characterization techniques, atomic force microscopy and confocal microscopy.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2017;5(3):031006-031006-8. doi:10.1115/1.4036890.

The manufacture of micro–nano structures in transparent dielectrics is becoming increasingly important due to the applications in medical and biological sciences. The femtosecond pulsed laser, with its selectivity, high precision, and three-dimensional direct writing nature, is an ideal tool for this processing technology. In this paper, an improved model for the prediction of ablation crater shape and fluence threshold in femtosecond laser processing of fused silica is presented, in which self-trapping excitons and electrons' relaxation are involved to depict ionization process, Thornber's and Keldysh's models are employed to estimate ionization rate precisely, and a novel ablation criterion is proposed to judge ablation. Moreover, the relationship between the ablation fluence threshold and laser pulse duration is investigated with three different extrapolation methods. The results indicate that no matter which extrapolation method is employed, the ablation fluence thresholds predicted by the presented model agree with the published data.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2017;5(3):031007-031007-12. doi:10.1115/1.4036933.

This paper reports the development of an original design of chip breaker in a metal-matrix polycrystalline diamond (MMPCD) insert brazed into a milling tool. The research entailed finite element (FE) design, laser simulation, laser fabrication, and machining tests. FE analysis was performed to evaluate the effectiveness of different designs of chip breaker, under specified conditions when milling aluminum alloy (Al A356). Then, the ablation performance of an MMPCD workpiece was characterized by ablating single trenches under different conditions. The profiles of the generated trenches were analyzed and fed into a simulation tool to examine the resultant thickness of ablated layers for different process conditions, and to predict the obtainable shape when ablating multilayers. Next, the geometry of the designated chip breaker was sliced into a number of layers to be ablated sequentially. Different ablation scenarios were experimentally investigated to identify the optimum processing conditions. The results showed that an ns laser utilized in a controllable manner successfully produced the necessary three-dimensional feature of an intricate chip breaker with high surface quality (Ra in the submicron range), tight dimensional accuracy (maximum dimensional error was less than 4%), and in an acceptable processing time (≈51 s). Finally, two different inserts brazed in milling tools, with and without the chip breaker, were tested in real milling trials. Superior performance of the insert with chip breaker was demonstrated by the curled chips formed and the significant reduction of obtained surface roughness compared to the surface produced by the insert without chip breaker.

Commentary by Dr. Valentin Fuster
J. Micro Nano-Manuf. 2017;5(3):031008-031008-8. doi:10.1115/1.4036891.

Molecular dynamics (MD) simulations are performed to investigate the wettability of gold substrate interacting with nanosized droplets of water. The effects of droplet size, temperature variation, and impingement velocity are evaluated using molecular trajectories, dynamic contact angle, spread ratios, radial distribution function (RDF), and molecular diffusion graphs. Droplets of 4 nm and 10 nm were simulated at 293 K and 373 K, respectively. Stationary droplets were compared to droplets impinging the substrate at 100 m/s. The simulations were executed on high-end workstations equipped with NVIDIA® Tesla graphical processing units (GPUs). Results show that smaller droplets have a faster stabilization time and lower contact angles than larger droplets. With an increase in temperature, stabilization time gets faster, and the molecular diffusion from the water droplet increases. Higher temperatures also increase the wettability of the gold substrate, wherein droplets present a lower contact angle and a higher spread ratio. Droplets that impact the substrate at a higher impingement velocity converge to the same contact angle as stationary droplets. At higher temperatures, the impingement velocities accelerate the diffusion of water molecules into vapor. It was revealed that impingement velocities do not influence stabilization times. This research establishes relationships among different process parameters to control the wettability of water on gold substrates which can be explored to study several nanomanufacturing processes.

Commentary by Dr. Valentin Fuster

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