Micromanufacturing

This paper describes an investment casting process to produce aluminum molds having integrated microstructures. Unlike conventional micromolding tools, the aluminum mold was large and had complex curved surfaces. The aluminum was cast from curved microstructured ceramic molds which were themselves cast from curved microstructured rubber. The aluminum microstructures had an aspect ratio of 1:1 and sizes ranging from 25 to 50 μm. Many structures were successfully cast into the aluminum with excellent replication fidelity, including circular, square and triangular holes. We demonstrate molding of large, curved surfaces having surface microstructures using the aluminum mold. (Some figures in this article are in colour only in the electronic version)

This paper describes casting-based microfabrication of metal microstructures and nanostructures. The metal was cast into flexible silicone molds which were themselves cast from microfabricated silicon templates. Microcasting is demonstrated in two metal alloys of melting temperature 70 • C or 138 • C. Many structures were successfully cast into the metal with excellent replication fidelity, including ridges with periodicity 400 nm and holes or pillars with diameter in the range 10–100 μm and aspect ratio up to 2:1. The flexibility of the silicone mold permits casting of curved surfaces, which we demonstrate by fabricating a cylindrical metal roller of diameter 8 mm covered with microstructures. The metal microstructures can be in turn used as a reusable molding tool. (Some figures in this article are in colour only in the electronic version)

Over the years, researchers have been working to mimic the nature by inducing superhydrophobic properties into a variety of material surfaces so that they exhibit non-wetting properties. Many diverse applications have been found in the fields, such as space and aerospace, defence, automotive, biomedical applications and engineering, sensors, apparels, and so on. Superhydrophobic surfaces repel water generally due to their surface texture or chemical properties. In this article, we focus on the functional applications of the superhydrophobic surfaces, and state-of-the-art fabrication technologies and processes, and the limitations of these processes to generate the superhydrophobic surfaces have been developed over the years.

This paper presents measurements and models for how the macroscopic curvature of microstructured polymers affects hydrophobicity. Flexible polymer substrates were fabricated with arrays of regular microstructures. The interaction of liquid drops with these surfaces was analyzed for flat substrates and substrates flexed into either positive or negative cylindrical shapes. Liquid droplet static contact angle and dynamic slide angle were measured for a range of surfaces. An increase in substrate curvature corresponded with decreased slide angle for liquid droplets suspended on the surface asperities. This phenomenon is investigated in terms of solid–liquid contact line and the periodicity of surface microstructures. We present a model that can be used to understand the observed phenomena and to design microstructure geometries for hydrophobicity. (Some figures in this article are in colour only in the electronic version)

In this report we investigate the effects of focused ion beam (FIB) machining at low doses in the range of 10 15 ions/cm 2 to 10 16 ions/cm 2 for currents below 300 pA on Si(100) substrates. The effects of similar doses with currents in the range 10pA to 300 pA were compared. The topography of resulting structures has been characterized using atomic force microscope, while crystallinity of the Si was assessed by means of Raman spectroscopy. These machining parameters allow a controllable preparation of structures either protruding from, or recessed into, the surface with nanometre precision.

Micromanufacturing has been the subject of intense research during the last couple of decades due to the growing trend of miniaturization. It has become necessary to manufacture features at the micro and nanoscale. It is a good inscribed book and it will assist the manufacturing society as an initial textbook on micro electro discharge machining (μ-EDM). This book comprises detailed coverage of μ-EDM from the history of the process to the latest evolutions in the form of μ-EDM variants. It is a good reference book for the researchers to explore the new findings. It begins with the introduction (Chapter 1) to μ-EDM, its requirements, general principles, history, macro-and μ-EDM capabilities, benefits, and challenges of μ-EDM technology. Subsequently, it emphasizes the need for miniaturization in the field of manufacturing, broadly the microfabrication processes which can be classified as subtractive, additive, deforming, and joining processes. The development in the field of μ-EDM process is discussed step by step with the help of suitable diagrams. The principle of μ-EDM is explained concisely. The distinction between macro-and μ-EDM is explained by the data of the process parameters and performance. The basic component of μ-EDM setup such as a pulse generator (resistor-capacitor (RC) and transistor type), servo system, dielectric flushing, measuring, and positioning system has been discussed. The advantages of μ-EDM over the other available micromanufacturing processes are enlisted. At the end, the challenges associated with μ-EDM have been discussed, which provide the pathway for researchers or industry personnel to work in the field of μ-EDM. Chapter 2 discusses key issues and errors associated with μ-EDM and they are elaborated along with their cause and effects. Analyzing the functions of different parts in a machine tool structure, their desired material properties are discussed. The functional requirements of the machine

This chapter focuses on metrology, sensors and controls for micromanufacturing. In general, a variety of sensors are employed in micromanufacturing. Many of them are used for metrology, and in particular dimensional metrology. Most are used on production machines to provide feedback during production. Few metrology systems for process and product control are available. The sensors and metrology systems available for product and process analysis are, in general, slow and not readily implemented. From a metrology and sensing perspective, the currently available systems are mostly two-dimensional in nature. Most of the measurement systems are fairly slow, which makes them suitable for R&D as their measurement speed limits their utility on actual production lines. Controllers fall into two architecture categories, open and closed (standard computer numerical control (CNC) controller architecture). In most cases, commercially available systems employ more traditional closed architecture controllers, while research and development teams tend to use open architecture systems. For most manufacturing systems, process control is a desired capability. In many cases it is achieved by measuring parameters from both the process and the product and adjusting process parameters to improve the quality of the part. In general, this approach is far from being achieved in the micromanufacturing field as the actual measurement of part and process parameters in realtime is limited to a few very special instances. Furthermore once a measurement is made, the process must be tuned according to the process model. In most cases, such models do not exist. Thus, in order to effectively achieve process control and quality enhancement, improved real-time metrology systems and sensors are needed along with models that accurately describe the various manufacturing processes at the micro level such that true process control can be enabled.

Résumé/Abstract Semisolid powder forming is a promising approach for near-net shape forming of features in macro-/microscale. In this paper, viscosity and phase segregation behavior of Al-Si powders in the semisolid state were studied with back extrusion experiments. The effects of process parameters including shear rate, extrusion ratio, heating time, and precompaction pressure were analyzed using the design of experiments method. The results showed that the effects of shear rate, extrusion, ratio and heating time were ...

This work presents evaluation of macro sized metal based microgrippers of various shapes such as circular, square and rectangular, which can hold circular, square and rectangular microcomponents respectively. The developed microgrippers can be used effectively as a holding device in mechanical environments and also in the assembly of microcomponents. Microgrippers are designed and analyzed using SOLIDWORKS and COMSOL Multiphysics respectively. Fabrication of microgrippers are carried out using direct metal laser sintering process with six gripping arms to prevent the slipping of the microcomponents during handling. The microgrippers were actuated by means of specially designed actuation system. The fabricated microgrippers are characterized for gripping displacements and gripping forces and the results are compared with that of the simulation and analytical results. It is observed that the displacements of the gripping arms of the fabricated microgrippers are four times higher than ...

The advancement of polymer moulding tools is increasingly focused on imparting not only form but also surface texture for functionality to the surfaces of parts that are created. Furthermore, the increasing demand for inexpensive and higher quality micro-components means that tools for replication processes must take advantage of advanced manufacturing techniques. Tools created by processes such as micro-investment casting, as in this case, may often suffer from excessive surface roughness, malformed edges and general deformation. This results in higher de-moulding forces and a reduction in fidelity of moulded parts to design intent. In this study, large-area electron beam irradiation (EB) is shown to be an effective technique for improving these metrics. For the first time, large population, high aspect ratio micro-features are subject to this process and the mechanisms of smoothing and key enhancement phenomena are demonstrated. The possibility of including EB irradiation in an integrated process chain for arriving at net shape is also discussed. Surfaces of protruding features are shown to have surface roughness reduced significantly from 126 to 22 nm Ra value, with bottom substrate also similarly improving from 150 to 27 nm Ra. Bottoms of recessed features are also observed to have much improved surface finishes. 'Doming' of tops of column features is also demonstrated, further enhancing form. These features would be far too fragile to be polished by any other mechanical method. Crown

In this report we investigate the effects of focused ion beam (FIB) machining at low doses in the range of 10 15 ions/cm 2 to 10 16 ions/cm 2 for currents below 300 pA on Si(100) substrates. The effects of similar doses with currents in the range 10pA to 300 pA were compared. The topography of resulting structures has been characterized using atomic force microscope, while crystallinity of the Si was assessed by means of Raman spectroscopy. These machining parameters allow a controllable preparation of structures either protruding from, or recessed into, the surface with nanometre precision.

This article on ‘Micro-machining: An Overview (Part II)’ is in continuation to ‘Micro-machining: An Overview (Part I)’ published in this journal ( Journal of Micromanufacturing). It consists of four parts, namely, electrochemical micro-texturing, electrochemical spark micro-machining, molecular dynamics simulation and sustainability issues of micro-machining processes. Electrochemical micro-texturing (ECMTex) deals with various techniques developed for micro-texturing on different types of workpiece-surfaces, namely, flat, curved and free-form surfaces. Here, basically two categories of techniques have been reviewed, namely, with mask and without mask. It also deals with ‘single point tool micro-texturing’ which turns out to be a single-step technique requiring minimum time, but the accuracy and repeatability obtained after micro-texturing need to be critically analysed. For mass production, one needs to go for sinking kind of ECMTex processes. Electrochemical spark micro-machining ...

This paper presents a comprehensive approach for measurement and analysis of radial throw and the associated tool-tip trajectory in micromachining when using ultra-high-speed (UHS) spindles. The effect of radial throw on micromachining accuracy could be significant due to the strict absolute-tolerance requirements. However, accurately determining radial throw in micromachining poses challenges due to the micron-scale tool dimensions and high rotational speeds. In contrast to run-out, radial throw depends on the tool-rotation angle and dictates the instantaneous position (trajectory) of the cutting edges. This work first presents a mathematical framework to obtain the radial throw at the cutting edges by measuring radial throw at two locations on the tool shank. A laser Doppler vibrometry-based experimental approach is then described to accurately measure the radial throw from the tool shank in two mutually-perpendicular directions. Next, the variations on radial throw measurements are evaluated, the effect of spindle speed on the radial throw is analyzed, and the predictions of tool-tip radial throw are compared with those directly measured from the tip of a micro-tool blank. A study is then performed to assess the effect of tool attachment/detachment cycles on radial throw. Lastly, a set of simulations is conducted to demonstrate how the radial throw magnitude and orientation affect the surface location error, sidewall surface roughness, and flute-to-flute chip-thickness variations. It is concluded that the presented mathematical framework and measurement approach can be used to determine the tool-tip radial throw magnitude with better than 3.5 % ±2% accuracy. Increased spindle speed considerably increases the radial throw, e.g., from 8 μm at 60,000 rpm to 16 μm at 130,000 rpm. The tool attachment/detachment induces less than ±6.9% variation to the mean radial throw magnitude at 60,000 rpm. The main contributor to the variation of the radial throw orientation is identified as the inherent inaccuracies of the spindle.

This paper describes a novel wafer bonding technique using microwave heating of parylene intermediate layers. The bonding is achieved by parylene deposition and thermal lamination using microwave heating. Variable frequency microwave heating provides uniform, selective and rapid heating for parylene intermediate layers. The advantages of this bonding technique include short bonding time, low bonding temperature, relatively high bonding strength, less void generation and low thermal stress. In addition, the intermediate layer material, parylene, is chemically stable and biocompatible. This bonding technique can be used for structured wafers also because parylene provides a conformal coating. Therefore, this is a very attractive bonding tool for many MEMS devices. The bonding strength and uniformity were evaluated using diverse tools. Fracture mechanisms and the effects of bonding parameters and an adhesion promoter were also investigated. The bonding with a structured wafer was also successfully demonstrated.

Semisolid powder processing (SPP) is a promising approach for near net-shape forming of features in micro/meso-scale. By combining the concept of forming in the semisolid state and conventional powder metallurgy, SPP provides a novel solution to various processing and materials engineering challenges faced in micromanufacturing. Replacing bulk materials with powdered materials adds a new dimension to the traditional semisolid technique by allowing tailoring of material properties. In this dissertation, experimental study to understand flow characteristics of metallic powders in the semisolid state is performed, and its potential application to the fabrication of a functionally graded structure (FGS) is demonstrated.

The viscosity and phase segregation behavior of Al-Si powders in the semisolid state were first studied with back extrusion experiments. Effects of process parameters including shear rate, extrusion ratio, heating time and pre-compaction pressure were analyzed using the design of experiments method. The results showed that the effects of shear rate, extrusion ratio and heating time were statistically significant factors influencing the viscosity. The semisolid state powders showed a shear thinning behavior. Moreover, microstructure analysis of extruded parts indicated severe phase segregation during the forming process. As the extrusion opening became small (~400 μm), the phase segregation increased.

A two-layer FGS with one layer reinforced by SiC particles was fabricated with SPP. The results indicate that SPP is capable of fabricating graded structures with promising microstructures and mechanical properties. When the SiC particles are larger than the matrix powder, dense and strong parts were formed. Smaller SiC particles can isolate the metal powders and result in porous and weak structures. The roughness of the SiC particle surface affects interface bonding between SiC particles and Al-Si-Cu matrix phase.

In summary, SPP has the potential to become a viable micromanufacturing method that can be used to make graded structures with low cost, good microstructure and promising properties.

Patterning a surface by scratching at micro-nano scale requires precise instrumentation strategies for generation of accurate patterns which are used in variety of surfaces ranging from bacteria resistant surfaces to water repellent ones. The scratching action leads to temperature rise of tool tip causing expansion of tool and work surface. This results in positioning inaccuracies and surface defects of the machined work. In this work a fiber Bragg grating sensor (FBG) is used to record the temperature rise at the tool tip under different scratching/cutting speeds. The surface integrity of the patterned work was evaluated after machining using a profilometer. The optimized value of cutting speed for minimum inaccuracy and minimum burr was found using Lagrangian multiplier based technique. This work thus ensures real time control of scratching speed based on tool tip temperature variations only, which finally results in better surface integrity of patterned profile.

Rotational-Magnetorheological Abrasive Flow Finishing (R-MRAFF) process is a technique of nano-finishing in general and free-form/sculptured surfaces in particular. Knee joint is one such example of free-form surface which is used by orthopedic surgeons. Knee joints are usually made of cobalt chromium alloy which is bio-compatible material, and it is difficult to finish. The present work deals with the development of an inverse replica for a knee joint. This inverse replica is used as a fixture so that the fluid can flow with constant velocity within the confined gap between the workpiece and inner surface of the fixture in order to achieve uniform nano-level finish. Using this technique, variation in the surface roughness ( Ra) over the knee joint surface can be minimized and productivity can be enhanced. The finishing efficiency of the R-MRAFF process depends on the input parameters such as extrusion pressure, number of finishing cycles, rotational speed of the magnet, etc. These ...

The use of micro and nanocomponents has increased rapidly over the last decade. The indications are that this trend is going to be sustained or even increase in the years to come due to the widespread use of these components in several micro and nanosystems. The development of such components and systems is supported by a wide range of CAD, CAM and CAE tools. These tools are either classic modeling methods already used in traditional manufacturing or modeling techniques specially developed for modeling and simulation in the micro or nanometer regime. In both cases special attention should be given to microscopic level phenomena that complicate the modeling process and increase the difficulty of applying these methods in this area. Modeling and simulation are definitely key tools employed by scientists and engineers in developing, testing, evaluating and analyzing the characteristics and performance of their micro and nanocomponents and systems. This paper reviews some of the most common techniques used in modeling and simulation of micro and nanomanufacturing; techniques such as the finite element method and molecular dynamics are analyzed. Furthermore, the application of these methods in several areas, namely micro and nanomanufacturing, nanomaterials and MEMS is discussed and the relative bibliography is presented.

Patterning a surface by scratching at micro-nano scale requires precise instrumentation strategies for generation of accurate patterns which are used in variety of surfaces ranging from bacteria resistant surfaces to water repellent ones. The scratching action leads to temperature rise of tool tip causing expansion of tool and work surface. This results in positioning inaccuracies and surface defects of the machined work. In this work a fiber Bragg grating sensor (FBG) is used to record the temperature rise at the tool tip under different scratching/cutting speeds. The surface integrity of the patterned work was evaluated after machining using a profilometer. The optimized value of cutting speed for minimum inaccuracy and minimum burr was found using Lagrangian multiplier based technique. This work thus ensures real time control of scratching speed based on tool tip temperature variations only, which finally results in better surface integrity of patterned profile.