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A Novel Laser Tool for High-Volume Sample Preparation Fast, clean, and efficient laser ablation for microstructure diagnostics Thomas Höche, Michael Krause, Martin Ebert and Uwe Wagner
Over the past fifty years, lasers have perpetuated to find new, often groundbreaking applications in science and technology. The most important features of lasers are that photons are inherently free of elemental contamination, extremely high energy densities can be focused in very small areas and the laser beam can be precisely positioned using deflection mirrors. By reducing pulse lengths from a few nanoseconds down to the picosecond or femtosecond range, materials ablation is becoming increasingly “athermal”, i. e., structure damage by local heating is reduced to well below a few microns. In view of these outstanding characteristics of lasers as tools for micromachining, it is very surprising that sample preparation for microstructure diagnostics so far hasn’t made use of laser technology. The all-new, patented laser-micromachining tool microPREP is the first instrument to make fast, clean, and efficient laser ablation available for the preparation of samples for microstructure diagnostics. Exemplified for a sample to be investigated by transmission electron microscopy (TEM) and following a three-stage approach, a supporting basic structure is cut from the feedstock first. Second, the supported structure is thinned down to a few microns of residual thickness and third, the supported and thinned structure is polished using an ion broad beam. Illustrated by numerous examples, it is shown that this technology is ready to be applied on different areas of microstructure diagnostics and has very high potential for failure diagnostics.
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Lasers as reliable tools for micromachining Being commercially available for many decades, lasers have become indispensable and extremely reliable tools in many areas of life, including automotive industry (welding, drilling, and measuring), thin-film processing (scribing, patterning), and even esthetic surgery (dermabrasive ablation, LASIK). It is most surprising that people do not have any objections towards laser vision correction, i. e., exposition of their eye – the most delicate human detector – to ablative laser light, while the preparation of samples for microstructure diagnostics has so far made only little use of laser micromachining. Things are getting even more inapprehensible when considering the favorable properties of lasers. Laser radiation can, sufficiently high power (or more precisely: fluences) provided ablate all kind of materials. Using ultrashort pulses and/or very high pulse energies, ablation is based on multi-photon absorption, enabling the machining of transparent-at-the-wavelength materials. The ablation rate of laser micromachining is about six orders of magnitude larger than that of a Ga+ focused ion beams used in FIB workstations for microstructure diagnostics sample preparation and still roughly three orders of magnitude higher than the milling rate of a recently introduced Xe+-PlasmaFIB. Beyond the aforementioned advantages, lasers can be precisely positioned on a given workpiece and straight-forwardly focused using standard optical elements. Since laser radiation consists just of photons – it is not a corpuscular radiation like ion beam that may tend to unwanted implantation effects – laser
Fig. 1 The microPREP tool is a compact desktop instrument. As the laser ray path is fully housed, it complies with the laser class 1 classification.
micromachining is very clean in terms of contamination.
Lasers meet preparation needs for microstructure diagnostics There is an increasing demand for preparation techniques tuned to manifold methods of microstructure diagnostics that need to be fast, reliable, cost effective, artifact-free, and targeted on the micron scale or beyond. Besides traditional mechanical preparation, focused-ion-beam micromachining is currently dominating the field. While the former is accompanied by high costs for skillful personnel, the latter is characterized by very high costs of ownership. In this contribution, we present the capabilities of an all-new tool for laser-based preparation illustrated by a number of examples. Based on several patents – e. g. by [1, 2] – the microPREP tool (Fig. 1) has a basic functionality that consists of laser cutting of a base structure followed by local laser thinning in an almost entirely automated
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Laser Materials Processing
Fig. 2 TEM cross-section of a flank of laser-micro-machined silicon showing the extent of crystallized slabs to be limited to less than a few 100 nm.
fashion. Making use of a rugged, ultrashort-pulse laser source, the process is characterized by very low running costs, suitability for semiconductors, metals, ceramics, as well as compounds thereof, and a very high targeted precision on the one-digit micron scale.
Heat-affected zone Widespread use of laser micromachining for the preparation of microstructure-diagnostics samples has been hampered by concerns regarding thermal damage to the sample. A few points need to be considered in order to understand that this effect can be controlled appropriately. The interaction of shortpulsed (not ultrashort-pulsed) laser radiation with matter is dominated by the formation of a melt and expelling the latter from the location of machining. This is because the electromagnetic wave couples to the phononic system, and the excitation of phonons leads to a local increase of temperature. For ultrashort pulsed lasers, however, due to non-linear optical effects, the excitation is attained via the elec-
Fig. 4 SEM micrograph of a supported-bar basic structure laser-machined into silicon.
tronic system of the materials, leading to much reduced heat effects. Ablation is not entirely athermic, like postulated in the beginnings of machining with ultrashort pulsed lasers, but it proved to be restricted to less than a few 100 nm [4 – 6]. Moreover, the extent of this heat-affected zone (HAZ) can become larger when basic principles of laser micromachining, like utilization of a too low feed rate, are not obeyed. Ultimately, one has to face the fact that there is a certain layer of material on a machined surface that will contain structural damage. For the ultrashort pulsed used in microPREP, a typical example of laser-micromachined silicon is depicted in Fig. 2.
Basic workflow The basic workflow detailed below was developed for bulk samples. Beyond this, optional extensions towards the processing of cross-sections of functional structures, tomography and atom-probe pillars, micromechanics test samples, etc. are under development. The basic workflow consists of six steps: (1) Plane-parallel thinning to 100 – 150 µm, (2) Assembling the plate on a flat mount, (3) Laser cutting of the base structure, (4) Transferring the base structure to a dedicated clamping jig, (5) Local laser thinning of preselected areas (single- or both-sided), (6) Final, single- or both-sided thinning, either with an Ar+ ion broad beam or with a focused Ga+ or Xe+ ion beam. In the sixth and final step, structural damage in the HAZ will be readily removed leaving an electron-transparent sample.
Fig. 3 Illustration a few selected basic structures that can be machined with the laser. One cannot just make any given geometry fitting into standard samples holders, but can also realize stabilizing bars and even marking for sample tracing.
Unequaled flexibility helps fulfilling manifold demands In steps (3) and (5), one can make use of a virtually unlimited flexibility in terms of laser trajectories. Just regard the laser as a tool of about 12 µm diameter that can cut out every imaginable structure (Fig. 3). It is not just that you can design and cut a supporting structure to exactly fit the needs for successive characterization of the microstructure (like tips, bars, etc.), also the second step of the laser-based preparation (the thinning of the supported structure) offers an unequaled choice of patterns to micromachine. But also for local thinning, flexibility is endless. Lined up with ion-beam thin-
Company 3D-Micromac AG
Chemnitz, Germany
3D-Micromac develops and manufactures highly efficient state-of-the-art laser micromachining systems and customized solutions e. g. for industrial applications in photovoltaics, semiconductor processing, medical device technology. These machines can be designed as highly precise stand-alone-systems or integrated solutions for existing, fully automatic production lines. 3D-Micromac has a fully equipped application laboratory with experienced application and process engineers to support customers in both process development and establishment as well as in selecting the appropriate machine configuration. Since its founding the company has gained an established position in international markets with sales and service partners in North America, Asia, and Europe. www.3d-micromac.com
Fig. 5 SEM micrograph of a supported-bar basic structure in silicon after local thinning in a finger-like manner.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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The modular stage and software design will allow to meet the growing requirements of clients and the market.
ning, electron transparent sections cannot just be made at one position of your choice, but at multiple positions, allowing for high-throughput screening of the microstructure. This feature is particularly valuable for gradient materials and if the homogeneity of a given microstructure needs to become evaluated. Making use of these features allows the operator to exactly tune the sample preparation to the requirement of microstructure diagnostics.
DOI: 10.1002/latj.201500002
Application examples Laser micromachining of silicon
In Fig. 4, a supported bar is shown after step (4) of the workflow. In Fig. 5, local thinning is demonstrated for the supported-bar basic structure shown in Fig. 4. Apparently, the imagination of the operator in terms of how to arrange the thinning pattern is hardly limited.
Outlook Beyond its basic functionality, Fig. 6 demonstrates that microPREP is capable of fabricating tiny pillars in an automated fashion that could, for example, be used as mounts or basic structures for atom-probe tomography.
Fig. 6 SEM micrograph of a pillar machined into silicon.
Conclusions Laser-based sample preparation is opening up new approaches for fast, cost-effective, and site-specific preparation of samples for microstructure diagnostics. With microPREP, laser micromachining for manifold tasks related to microstructure diagnostics is always at your fingertips. It is not just a tool to prepare samples for transmission electron microscopy but will come with options for atom-probe microscopy, tomography, transmission Kikuchi diffraction, and micromechanical testing.
[1] Th. Höche: PCT Patent Application WO 2013/026707 A1 [2] M. Krause, Th. Höche: EP Patent Application EP 13 16 2360. [3] Th. Höche, D. Ruthe, T. Petsch: Femtosecond-Laser Interaction with Mo/Si Multilayer Stack at Low Fluence, Appl. Phys. A 79 (2004) 4-6, 961. [4] Th. Höche et al.: Nanostructural Investigations on Ripples Prepared by Femto-Second Laser Treatment of Silicon, Proc. 2nd International Congress on Applications of Lasers & Electro Optics (ICALEO), Jacksonville, FL, October 2003. [5] D. Ruthe, K. Zimmer, Th. Höche: Etching of CuInSe2 Thin Films – Comparison of Femtosecond and Picosecond Laser Ablation, Appl. Surf. Sci. 247 (2005) 447. [6] S. Martens et al.: Simulation-Based Analysis of the Heat-Affected Zone during Target Preparation by Pulsed-Laser Ablation through Stacked Silicon Dies in 3D Integrated System-in-Packages, 11th Int. Conf. on Thermal, Mechanical and Multiphysics Simulation and Experiments in Micro-Electronics and Micro-Systems, EuroSimE, 2010.
Authors Thomas Höche
received a PhD in materials science from the University of Stuttgart. He has been applying microstructure diagnostics to glasses, glass ceramics, and other inorganic materials at the University of Jena and Humboldt University Berlin. In 2002, he joint Leibniz IOM and worked in parallel for 3D-Micromac AG of Chemnitz. After completing his habilitation in 2005, Höche became Extraordinary Professor for experimental physics in 2008. From 2010 on, Höche pursues research on nanomaterials and nanoanalytics at Fraunhofer IWM in Halle.
Michael Krause
studied physics with a focus on surface sciences at Martin-LutherUniversity Halle-Witten berg and completed his diploma thesis on surface mechanics in 2006. In the same year he joined Fraunhofer Institute for Mechanics of Materials IWM in Halle as a PhD student. Since completion of his doctorate in 2013 he works as a Project Manager as well as Team Manager for surface analytics.
Martin Ebert
studied information technology with focus on semiconductor technology and sensor systems at the University Magdeburg and completed his diploma thesis in 2011. Since 2012 he works as engineer in the field of SEM/FIB analytics and laser technology at the Fraunhofer Institute for Mechanics of Materials IWM in Halle.
Uwe Wagner
joined 3DMicromac in 2012 and is now Chief Sales Officer & Business Development Manager. After graduation in communication technology, he worked as a scientific engineer with the Laser Institute Hannover, LZH. In 1999, he accompanied with LPKF Laser & Electronics AG the buildup of a new business segment of UV laser machines for PCB manufacturing. Starting in 2005, as the Head of global sales, he orchestrated a new business segment with Jenoptik Automatisierungstechnik and set in train the Business Development for Jenoptik Laser & Material Processing Division.
3D-Micromac AG, Mandy Gebhardt, Marketing & Public Relations,Technologie-Campus 8, D-09126 Chemnitz, Germany, Tel.: +49 (0)371 400 43-0, E-Mail:
[email protected]
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