Laser-induced forward transfer (LIFT) of 3D microstructures

Accurate replications of complex, high aspect ratio nano and micro structured parts are still challenging due to the comparatively high surface-volume ratio. The critical process step of automated, undistorted demoulding in high precision replication techniques like hot embossing or thermal nano imprint require good process control at elevated temperatures just below the solidification of the thermoplastic material and higher adhesive forces of the polymer part to the substrate plate than to the structured tool insert. The required increase in interfacial surface to the substrate plate is typically done by a rough substrate plate which results in milky, in-transparent residual layer. We demonstrate a process modification which keeps the advantage of precise automated demoulding and allows for replication of micro structured parts with optically transparent residual layer even for high aspect ratio structures resulting in high demoulding forces.

Laser induced photo-polymerization is rapid prototyping technique for the manufacturing of high aspect ratio micro- mechanical parts. The process consists of UV laser-induced polymerization of multifunctional monomers. This photo- forming method has advantages of speed and economy over other methods used in the manufacturing of high aspect ratio microstructures. However, current photo-forming systems do not yet have the accuracy and resolution required for micro- mechanical fabrication. To achieve a system with high resolution, it is desirable to develop a predictive mathematical model that can be used in process optimization. In this study, we develop a mathematical model that predicts the laser beam pixel dwell time (or the scanning scheme) on the monomer layer for attaining uniform monomer conversion or solidification by which one can achieve dimensional accuracy and smooth surfaces of the microstructure.

Superhydrophobic surfaces combine high aspect ratio micro- or nano-topography and hydrophobic surface chemistry to create super water-repellent surfaces. Most studies consider their effect on droplets, which ball-up and roll-off. However, their properties are not restricted to modification of the behaviour of droplets, but potentially influence any process occurring at the solid-liquid interface. Here, we highlight three recent developments focused on the theme of immersed superhydrophobic surfaces. The first illustrates the ability of a superhydrophobic surface to act as a gas exchange membrane, the second demonstrates a reduction in drag during flow through small tubes and the third considers a macroscopic experiment demonstrating an increase in the terminal velocity of settling spheres.

HARMST 2009 was held from June 25 to 28, 2009, in Saskatoon, Saskatchewan, Canada. This workshop continued a biennial tradition that began in 1995 in Karlsruhe, Germany, and travels around Europe, America, and Asia, successively held in Madison, WI, USA (1997), Chiba, Kisarazu, Japan (1999), Baden-Baden, Germany (2001), Monterey, CA, USA (2003), Gyeongju, Korea (2005), and Besancon, France (2007). This series has established itself as a premier, independent international event devoted to the advancement of high aspect ratio micro fabrication technologies and their applications. HARMST 2009 again brought together experts from academia and industry from various disciplines and regions who shared their latest results of investigations and developments. The technical program included sessions on a broad range of aspects of design, modeling, fabrication, metrology, testing, and applications in high aspect ratio microand nanotechnology. Particular focus was cast on X-ray and UV-lithography in thick resist layers, using various resist systems and including nonconventional processes, and subsequent electroforming and replication steps, as well as related simulation, modeling, and metrology techniques. Furthermore, selected advancements in DRIE, mechanical micromachining, and electro discharge micromachining relevant to high aspect ratio technologies were presented, as well as highlights of the integration of nanometer-scale structures, advanced materials, and functionalized surfaces. Applications were another focus of the workshop, from sensors and actuators to RF MEMS, MOEMS, fluidic microsystems, and bio-MEMS. The workshop also included a focus session on challenges and opportunities in micromechanics and microfluidics to discuss a cross section of aspects at the example of a few applications, including industrial needs, fabrication technology capabilities and restrictions, metrology, and testing. HARMST 2009 featured 74 poster and 39 oral presentations from 15 countries as well as 8 short course contributions. The HARMST workshops have been highly successful and continue their tradition not only in terms of presentation and discussion during the workshop, but also with respect to the timely dissemination of cutting edge technologies via the Journal of Microsystem Technologies. This Special Issue of Microsystem Technologies is collecting 53 finally accepted contributions as reviewed and unabridged versions of papers presented at the workshop, following the traditional cooperation between Springer and the HARMST Technical Advisory Committee. We hope that you will enjoy reading the HARMST contributions as much as we did.

With rapid development in micromachining, high aspect ratio (AR > 10) micro-holes are essentially required in applications such as spinneret holes, biomedical needles, fuel injectors, cooling channels, etc. Machining such high aspect ratio micro holes on superalloys are challenging for micromachining processes. A high aspect ratio micro hole can be machined via. Micro Electrical Discharge Machining (MEDM) process with optimum levels of operating parameters. Work reported in this paper is aimed to investigate the effect of machining parameters of MEDM to produc micro-holes in Hastelloy C276. Extensive experimentations based on Taguchi Designs of Experiments (DOE) were conducted to identify the most influential processing parameters while machining micro holes of more than 10 aspect ratios. The experimental results are discussed which quantify an individual and combined effect of different operating parameters on the overall performance of MEDM like material removal rate (MRR) and tool wear rate (TWR). It is found that capacitance and spindle rotation speed are the most important parameters controlling the stability of the process. A direct relation of TWR has been found with discharge energy (capacitance and voltage). At optimum process parameter conditions, the aspect ratio (depth/diameter) of 12.5 is achieved while machining holes with 400 µm diameter.

This research work deals with the methods for manufacturing high aspect ratio micro mechanical parts by using UV photopolymer. In the first method, polymer structures are produced using a mask-based method. The second method is a direct focused UV laser beam writing method. The irradiated portion of the liquid photopolymer is solidified and it becomes a polymer structure. At first, the solidified shape of the polymer is examined in each method and the optimum conditions to produce high aspect ratio structures are verified. Next, the comparison of both methods is made. Examples of structures that have been produced using the methods described are shown.

A simplified model for dark-field optical imaging of three-dimensional high aspect ratio micro- and nano- structures is proposed, to reduce the time taken to simulate object fields with in-plane scattering between different parts of the object. Primary scattering is found by assuming that illumination of Manhattan geometries generates a set of spherical edge waves, following the incremental theory of diffraction. Secondary scattering is found by assuming that primary scattering is re-scattered from nearby features. Diffraction coefficients are simplified, and the number of illuminating beams is limited to those generating waves that enter the objective lens. Images obtained using TE and TM polarizations are compared, and results are benchmarked against a vectorial finite element model. Applications lie in simulating optical inspection of structures containing vertically etched features including MEMS and NEMS.

The use of laser induced forward transfer (LIFT) techniques for printing materials for sensor and electronics applications is growing as additive manufacturing expands into the fabrication of functional structures. LIFT is capable of achieving high speed/throughput, high-resolution patterns of a wide range of materials over many types of substrates for applications in flexible-hybrid electronics. In many LIFT applications, the use of a sacrificial or laser-absorbing donor layer is required despite the fact that it can only be used once. This is because the various types of release layers commonly in use with LIFT are completely vaporized when illuminated with a laser pulse. A better solution would be to employ a reusable laserabsorbing layer to which the transferable ink or material is attached and then released by a laser pulse without damage to the absorbing layer, therefore allowing its repeated use in subsequent transfers. In this work, we describe the use of two types of reusable laser-absorbing layers for LIFT. One is based on an elastomeric donor layer made from poly(dimethylsiloxane) or PDMS, while the other is based on a ceramic thin film comprised of indium tin oxide (ITO). These release layers have been used at NRL to transfer a wide range of materials including fluids, nanoinks, nanowires and metal foils of varying size and thickness. We will present examples of both PDMS and ITO as donor layers for LIFT and their reusability for laser printing of distinct materials ranging from fluids to solids.

In our work, we suggest a simple one-step method for the fabrication of slanted surface structures. Spikes - self-organized, quasi-periodical arrays of high aspect ratio micro structures - can be fabricated e.g. by laser irradiation of silicon (Si) surfaces in a reactive gas atmosphere (e.g. SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> ). Moreover, spikes generation was reported on a wide range of materials, including semiconductors and metals. The geometrical parameters of spikes (their dimensions and density) are mainly controlled by changing the incident laser fluence (laser pulse energy per unit area). Already in the first reports dealing with the generation of spikes, it was established that the spikes always point in opposite direction of the incident laser beam. Therefore, it is possible to generate slanted spikes by mounting samples at different angles to the incident laser beam. We use this method to fabricate slanted spike microstructures.

Ultra-tall (2–4 mm), densely packed arrays of high aspect ratio micro structures (HARMs) are required for a variety of heat transfer and mass transfer devices currently under development. Great success has been reported lithographically defining relatively tall features using SU-8 as an x-ray resist, but excellent results are scarce with respect to patterning ultra-tall features with tight packing and small gaps between features. Diffusion of a cross-linking species from exposed regions to unexposed regions is believed to be the primary mechanism that prevents such features from being defined. In a companion paper, solvent content is shown to be an extremely important parameter that controls the quality of ultra-tall, tightly packed micro structure arrays. In this paper, a simple quasi-two-dimensional model is developed to qualitatively show that the adverse effects of diffusion of a cross-linking species can be significant. Also, experiments were performed to isolate the effects of parameters other than solvent content on performance (post exposure bake temperature, dose and top-to-bottom dose ratio, and feature height). These effects are placed in the context of the general hypothesis that preventing diffusion of cross-linking species is a key to successfully defining ultra-tall, tightly packed SU-8 HARMs. Experimental results are provided that demonstrate the ability to define SU-8 features with heights of 3 mm, and gaps between adjacent features of 125 µm (the gap therefore having an aspect ratio of 24).

Micro extrusion of high aspect ratio bi-lumen tubes using 17-4PH stainless steel feedstock

Micro-electrical discharge machining (EDM) is among the most important machining techniques for creating micro features, notably for cutting challenging materials such as superalloys, ceramics, and composite materials. To improve machining performance characteristics, micro-EDM technology was integrated with the mixing of marble powder in the dielectric fluid in this research. This work investigates the potential of marble powder mixed micro-EDM (PM- EDM) to manufacture high aspect ratio micro holes in a newly designed ceramic, TiN-Al2O3. SEM photos of the action of marble powder on the machined surface are also examined, with thermal cracking (Spalling) and melting being the most prominent mechanisms of material removal.

We report the successful fabrication and application of a micro-scale hybrid liquid wicking structure in flat polymer-based heat spreaders to improve the heat transfer performance under gravitational acceleration. The hybrid wick consists of 100 μm high, 200 μm wide square electroformed high aspect ratio copper micro-pillars with 31 μm spacing for liquid flow. A woven copper mesh with 51 μm diameter and 76 μm spacing was bonded to the top surface of the pillars to enhance evaporation and condensation heat transfer. The exterior device geometry is 40 mm × 40 mm × 1.0 mm. The 100 μm thick liquid crystal polymer (LCP) casing contains a two-dimensional array of copper filled vias to reduce the overall thermal resistance. The device was tested with heat flux input of up to 63 W/cm2 at horizontal and vertical orientations. The difference in temperature between the evaporator and condenser was measured and compared to a copper reference block of identical exterior dimensions. The experimentally determined thermal resistance of the copper block remained nearly constant at 1.2 K/W. The thermal resistance of the flat polymer heat spreader at horizontal orientation was 0.55 K/W. The same device at −90° adverse orientation resulted in a thermal resistance of 0.60 K/W. These measurements indicate that this hybrid wicking structure is capable of providing a capillary pumping pressure that is effective at transferring at least 63 W/cm2 heat flux regardless of orientation. This work illustrates an important step to developing more effective thermal management strategies for the next generation of heat generating components and the possibility of developing flexible, polymer-based heat spreaders fabricated with standardized printed circuit board technologies.

Infrared extinction, scattering and absorption coefficients have been theoretically calculated for high aspect ratio micro and nano particles of several highly conducting metals. Two particle geometries have been modelled, thin disc flakes and thin fibres. Optimum particle minor dimensions on the nano scale for maximum infrared extinction and attenuation performance have been predicted for these two geometries, and the relative performance of different metals has been evaluated. Particle phase functions and polarization states of the scattered light have also been calculated, and their dependence on particle orientation has been studied. The extinction coefficient calculations have been performed using the infinite cylinder solution and the finite difference time domain (FDTD) method. Transmission and attenuation calculations were performed for various scenarios using the Beer?Lambert law, and the six-flux and spherical harmonics methods of solving the full radiative transfer equation (RTE) for an absorbing and scattering medium. Of the metals studied, aluminium and brass particles are predicted to be the best potential attenuators of infrared radiation. The maximum possible volume extinction coefficients and required particle dimensions are calculated for aluminium and brass particles of both geometries.

Although high aspect ratio micro and nanoscale polymer features have been replicated in a range of polymers using injection molding, researchers have also used tooling inserts with a range of sizes, aspect ratios, and tooling materials. In this work, microscale features with molded in polymethylmethacrylates using three types of tooling with similar features. The tooling materials included silicon wafers with an antistiction coating, gold-coated nickel inserts, and a metal-polymer hybrid tooling. Tooling was evaluated based on the ease of melt filling and part ejection; the replication quality as characterized using optical profilometry, confocal microscopy, and scanning electron microscopy; and the damage to the tooling after repeated use. With lower aspect ratio features, the tooling type did not significantly affect replication, but for higher aspect ratio features the hybrid tooling provided far better replication than the silicon tooling. This difference was attributed to retardation of heat transfer in the features of the hybrid tooling. All three tooling materials exhibited polymer-free surfaces after injection molding.