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This article is the second in a four-part series of articles providing an overview of the challenges and opportunities in microscale additive manufacturing (AM) processes with applications in fabrication of high precision micro/nano-products. Laser-based microscale additive manufacturing processes are discussed this article. Compared to the other AM processes, laser-based processes provide several unique advantages, especially in terms of a wide variety of processable materials and high volumetric throughputs. The processes discussed in this paper can fabricate complex microscale features with minimum resolutions ranging from hundreds of nanometers to hundreds of microns. However, there are several fundamental limits and trade-offs which hinder the scalability of these processes. The paper discusses the limits to the materials, resolution, geometry, and volumetric throughput and proposes approaches to mitigate these limits and improve the scalability of laser-based microscale AM processes.

Microscale additive manufacturing is one of the fastest growing areas of research within the additive manufacturing community. However, there are still significant challenges that exist in terms of available materials, resolution, throughput, and ability to fabricate true three-dimensional geometries. These challenges render commercialization of currently available microscale additive manufacturing processes difficult. This paper is the last one in a four-part series of articles which review the current state-of-the-art of microscale additive manufacturing technologies and investigate the factors that currently limit each microscale additive manufacturing technology in terms of materials, resolution, throughput, and ability to fabricate complex geometries. Parts I, II and III offer prognoses about the future viability and applications of each technology along with suggested future research directions that could be used to bring each process technology in line with its fundamental, physics-based limitations. This paper brings together the general design guidelines that must be followed while designing scalable microscale AM processes. Finally, the paper concludes with an analysis of the role of precision engineering in the future advancement of microscale additive manufacturing technologies. This series of publications is a joint effort by the members and affiliates of the Micro-Nano Technical Leadership Committee of the American Society for Precision Engineering (ASPE).

This paper presents the development of a metrology framework and proof-of-concept tool to perform direct, nanometer-scale topography measurements for real-time process control in the roll-to-roll manufacturing of nanopatterened materials, films, and flexible electronic devices. The system leverages a uniquely compact single chip atomic force microscope (sc-AFM) based on a micro-electromechanical system (MEMS) architecture with custom approach mechanism positioned on a gantry which is actuated by two vertical, double parallelogram flexure mechanism guided two-axis nanopositioners. This probe is situated over a stainless-steel, air bearing supported idler roller and its position dynamically compensated for curvature, eccentricity and surface topography of the roller during web movement through the system from an offline map. The proof-of-concept tool performs a single, 400 μm2, non-contact tapping mode scan with nm height resolution on a flexible, 150 μm × 350 mm polycarbonate substrate every 60 s in a step-and-scan fashion where web movement occurs during each step. The performance of the sc-AFM gantry nanopositioning system is evaluated and a representative nanofeatured flexible material, the wing of a Queen Butterfly, is used as a test artifact to demonstrate the efficacy of the nanometrology data acquired. This capability represents a wholly new method for in-line metrology in roll-to-roll nanomanufacturing.

This paper seeks to demonstrate the efficacy of a new approach for fast, in-line, and direct topography measurement of nano-scale structures and features on a flexible substrate, or web, in a roll-to-roll fashion. Nanofeatured products manufactured with R2R processes can be extremely cost competitive compared to more traditional, wafer-based solutions in addition to their unique and desirable mechanical properties. As such they are an area of immense research interest. But, despite the promise of these products for a plethora of applications, the leap from lab-scale prototypes to pilot- or volumescale manufacturing has proven extraordinarily difficult and expensive — with both required research and development investment and achievable process yield proving sizable barriers. A key capability gap in current art and roadblock on the path towards more widespread research and adoption of these R2R fabricated products is the lack of high-throughput, nanometer-scale metrology for process development, control, and yield enhancement. In this work a new type of extremely compact, tip-based microscope designed and fabricated with a micro-electro-mechanical system approach is applied to the challenge of direct topography measurement for roll-to-roll fabricated nanopatterns. A proof-of-concept tool with subsystems to regulate the flexible web, isolate and position the atomic force microscope, and measure features on the substrate, all coordinated by a real-time embedded control system is shown and step-and-scan measurement results were acquired. However, to genuinely meet this extent need for roll-to-roll metrology, a system capable of atomic force microscope scanning despite a continuous, non-zero web velocity must be developed to meet throughput requirements without degrading measurement quality and thus help to enable the next generation of R2R nanomanufacturing technology.

This work describes the performance improvement of a polymer electrolyte fuel cell with a novel class of microporous layers (MPLs) that incorporates hydrophilic additives: one with 30 μm aluminosilicate fibers and another with multiwalled carbon nanotubes with a domain size of 5 μm. Higher current densities at low potentials were observed for cells with the additive-containing MPLs compared to a baseline cell with a conventional MPL, which correlate with improvements in water management. This is also observed for helium and oxygen experiments and by the lower amount of liquid water in the cell, as determined by neutron radiography. Furthermore, carbon-nanotube-containing MPLs demonstrates improved durability compared to the baseline MPL. Microstructural analyses including nanotomography demonstrate that the filler material in both the additive-containing MPLs provide preferential transport pathways for liquid water, which correlate with ex situ measurements. The main advantage provided by these MPLs is im...

Synchrotron x-ray imaging techniques, like x-ray computed tomography (CT) and radiography have proven instrumental in expanding the communities knowledge of complex transport and reaction kinetics in electrochemical devices such as fuel cells and electrolyzers. This work presents the development of novel x-ray CT imaging techniques for operando visualization of water within low temperature fuel cells at spatial resolutions spanning the micro and nano scales. The design of operando sample holders, for both micro x-ray CT and nano CT experiments is described in depth, and prototypes of these sample holders were evaluated across a set of requirements, the most important of which are x-ray transmissibility, electrical conductivity and mechanical stability. Water segmentation from micro x-ray CT data was enabled by an image subtraction method, where the image without water is subtracted from the one with water. Through iterative experimentations, the operando nano CT cell was developed to optimize mechanical compression, electric conductivity and gas flow. While three-dimensional fuel cell reconstructions were shown possible, there remain challenges to overcome at typical lower energies (8 keV) due to beam damage, whereas it is not as significant for higher energies (>17.5 keV).

The development of accurate, noncontact surface imaging is key to implementing an effective metrology strategy to manage defect detection in high volume flexible electronic device fabrication. This paper presents the design of a compound, double parallelogram flexure-hinge mechanism (DPFM) based nanopositioning system with stacked coarse-fine adjustment DPFMs. In concert with novel Atomic Force Microscope (AFM)-on-a-chip technology, this coupled, multi-flexure positioning system is proposed as a probe-based metrology device for roll-to-roll (R2R) electronics manufacturing and shown in Fig. 1 [1], [2]. The structural parameters of this system have been designed to ensure the desired stiffness, range of motion, and resonant modes are achieved. The parametric design of this positioning system has been verified through Finite Element Analysis (FEA). The proposed system will achieve a scanning throughput of six, 60 μm line scans every 0.15 seconds for a total throughput of over 75 μm2/s at a lateral resolution nearing 50 nm and a vertical resolution of less than 20 nm. This will allow for the development of a statistical metrology framework to reliably measure and analyze nanofeatured, R2R manufactured, flexible electronics in a cost-effective manner and provide fast, continuous defect identification.

Effective measurement of fabricated structures is critical to the cost-effective production of modern electronics. However, traditional tip-based approaches are poorly suited to in-line inspection at current manufacturing speeds. We present the development of a large area inspection method to address throughput constraints due to the narrow field-of-view (FOV) inherent in conventional tip-based measurement. The proposed proof-of-concept system can perform simultaneous, noncontact inspection at multiple hotspots using single-chip atomic force microscopes (sc-AFMs) with nanometer-scale resolution. The tool has a throughput of ∼60 wafers / h for five-site measurement on a 4-in. wafer, corresponding to a nanometrology throughput of ∼66,000 μm2 / h. This methodology can be used to not only locate subwavelength “killer” defects but also to measure topography for in-line process control. Further, a postprocessing workflow is developed to stitch together adjacent scans measured in a serial fashion and expand the FOV of each individual sc-AFM such that total inspection area per cycle can be balanced with throughput to perform larger area inspection for uses such as defect root-cause analysis.

© 2016 Elsevier B.V. There is a need to understand the structure properties of gas-diffusion layers (GDLs) in order to optimize their performance in various electrochemical devices. This information is important for mathematical modelers, experimentalists, and designers. In this article, a comprehensive study of a large set of commercially available GDLs' porosity, tortuosity, and pore-size distribution (PSD) under varying compression is presented in a single study using X-ray computed tomography (CT), which allows for a noninvasive measurement. Porosities and PSDs are directly obtained from reconstructed stacks of images, whereas tortuosity is computed with a finite-element simulation. Bimodal PSDs due to the presence of binder are observed for most of the GDLs, approaching unimodal distributions at high compressions. Sample to sample variability is conducted to show that morphological properties hold across various locations. Tortuosity values are the lowest for MRC and Freudenberg, highest for TGP, and in-between for SGL papers. The exponents for the MRC and Freudenberg tortuosity demonstrate a very small dependence on compression because the shapes of the pores are spherical indicating minimal heterogeneity. From the representative-elementary-volume studies it is shown that domains of 1 × 1 mm in-plane and full thickness in through-plane directions accurately represent GDL properties.

Effective liquid-water management is critical for optimal polymer-electrolyte fuel-cell (PEFC) operation. At lower operating temperatures and during startup, there is a need to remove liquid water from the cathode electrode to ensure reactant delivery1. At higher operating temperatures, large thermal gradients from catalyst layer to flow-field channel in the system promote water removal in the vapor phase due to the exponential dependence of water vapor pressure on temperature (i.e., phase-change-induced (PCI) flow). To achieve maximum water permeation and consequently higher current densities, it is necessary to understand the interplay between PCI flow, capillary-driven liquid water transport, and evaporation/condensation in PEFC porous diffusion media. PCI flow causes the liquid water to evaporate in hotter locations (electrode) and condense in cooler locations (PEFC’s land). Simultaneously, the heat redistribution occurs because heat is consumed during evaporation and released during condensation. Currently there is little understanding of how water is redistributed under PCI flow. Although water transport in diffusion media has been investigated with modeling and experiments,2-3 evaporation and PCI flow are poorly understood. The basic fundamental knowledge is lacking primarily due to a challenge of experimental measurements and visualization of the evaporating water front within these porous materials. In this talk, we will examine the water distribution and quantify local evaporation/condensation within diffusion media subjected to thermal gradients using synchrotron-based X-ray micro computed tomography (XCT). The novel experimental in-situ setup is presented that allows for precise temperature control and simultaneous water injection. Conditions studied include varying liquid pressure, thermal gradients, and GDL materials. Acknowledgments We thank Prof. Jeff Gostick for helpful insights and discussions. We thank Dr. Felix Büchi, Dr. Jens Eller and Dr. Adrien Lamibrac for experimental concept of evaporation experiment. This work was supported by EERE, Fuel-Cell Technologies Office, of the U.S. DOE under contract number DE-AC02-05CH11231. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. References 1. Weber, A. Z., et al., A Critical Review of Modeling Transport Phenomena in Polymer-Electrolyte Fuel Cells. J Electrochem Soc 2014, 161, F1254-F1299. 2. Zenyuk, I. V.; Parkinson, D. Y.; Hwang, G.; Weber, A. Z., Probing Water Distribution in Compressed Fuel-Cell Gas-Diffusion Layers Using X-Ray Computed Tomography. Electrochemistry Communications 2015, 53, 24-28. 3. Zenyuk, I. V.; Weber, A. Z., Understanding Liquid-Water Management in Pefcs Using X-Ray Computed Tomography and Modeling. ECS Transactions 2015, 69, 1253-1265.