Glass-based Manufacturing and Prototyping Platform PhotPack

Transcription

1 Glass-based Manufacturing and Prototyping Platform PhotPack New concepts for integrating various optical components in actively aligned glass assemblies Gunnar Böttger, Stefan Seifert and Henning Schröder Institute There has been considerable progress in utilizing fully automated machines for the assembly of micro-optical systems in recent years. Such systems integrate laser sources, passive optical elements, electro-optical components and detectors into tight packages, and efficiently couple light to waveguides in photonic integrated circuits (PIC), optical backplanes, free space or optical fibers. The required electro-optical and optical components are placed and aligned passively and actively, depending on optical and mechanical component tolerances. For one, the relative placing of all components is controlled actively by camera systems, and, more importantly, some or all active components are actually operated, and actively aligned with live feedback from internal and external power and beam image detectors. High precision (and also fast) active optical alignment machines nowadays exist, with more than adequate degrees of freedom and accuracies, so that best optical beam qualities and coupling efficiencies can be achieved. Fraunhofer Institute for Reliability and Microintegration IZM Berlin, Germany Fraunhofer IZM specializes in industry-oriented applied research, covering the entire spectrum of technologies and services necessary for developing reliable electronics, integrating new technology into industrial, automotive, healthcare and consumer applications. Consequently, more and more optical components are being integrated in hybrid or even all-optical assemblies, based on the existing expertise in the placing, joining and analysis of various microelectronic and MEMS components. electric wiring isolator FAC lens electric via thermal via air/fluid channels Fig. 1 Conceptual drawing of PhotPack, showing the main building blocks: Horizontal glass base plate stack with structures for added functionality (air/liquid or electrical/thermal interconnects or vias), and stacked holding elements with embedded light sources, optical components, fibers. PhotPack is an approach still under development at Fraunhofer IZM to make use of a complementary class of industrial production machines for effective photonic packaging: Laser processing machines that structure actual single or multi-level glass base plates and holding elements for aligning almost arbitrary optical components, or making optical components machine-placeable at all. IZM proposes using glass as an ideal base material for optical assemblies, adapting and integrating active and passive optical components in several respects. Traditionally, glass surfaces are being used in optical paths for imaging and beam shaping. But glass also shows distinct mechanical, electrical and thermal properties that should be made use of. Conventional processing of functional optical elements from bulk glass requires sawing, grinding, polishing or, more recently, replicating by using very accurate mechanical tools. These techniques are commonly multi-step procedures, and can only process a few optical elements per run. For mounting and a functional assembly, all optical components need framing, additional posts for leveling and/or structured substrate filters ball lens collimator optical fiber support through-holes (hermetic) to align all components to optical axes. Materials commonly used are precisely structured metals or ceramics which can be very expensive and have to be produced in volume, fixed designs and a limited number of runs to make them economically feasible. PhotPack differs in two main aspects: Firstly by using flat glass substrates or even drawn glasses in panel form as a starting material. Secondly, it provides glass support structures by using non-mechanical laser processing which does not require special tooling for further processing. Almost arbitrary 2D structures can thus be formed. To access the third dimension, we propose a discrete stacking and interleaving of 2D-lasered structures in a 2.5 D approach. This way, both mechanical-optical benches with 3D features, as well as supporting holding elements for optical components are created (see Fig. 1). By introducing special features, the latter ones are even alignable and fixable with six degrees of freedom (DOF). With ongoing progress in short-pulse laser ablating systems, at some point a direct and fast laser manufacturing in 3D might become feasible, but we have 42 Optik&Photonik 3/ WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2 Optical Components so far not found serious limitations with our very flexible and cost effective 2.5Ddiscrete approach in PhotPack. Optical surfaces or functionality in the future may also be created by additive manufacturing. To evaluate the assumed high potential of 3D-printed optical structures regarded as a possibly advantageous hybrid extension of all-glass Phot- Pack, this article toward the end includes a brief section on 3D-printed free form polymer structures and an initial analysis of the dimensional accuracy that can be reached for optically reflecting surfaces. Concepts and advantages of PhotPack With glass as the base material in Phot- Pack, dimensionally stable optical assemblies can be created. Most passive micro-optical components such as lenses, beam splitters, mirrors or filter elements are also made of glass, or are based on glass substrates, so a matching of thermal coefficients of expansion is straightforward. By PhotPack not only mechanical base plates can be designed, of easily adaptable and scalable size and complexity, but they can also be patterned with electrical wiring, structured with air and fluidic channels or thermal and electrical vias (compare Fig. 1). Furthermore, holding elements for individual or off-the-shelf functional components can be created so that, along with an active alignment strategy, an integration and adaption of very diverse (electro-) optical components becomes possible. The latter ones may differ widely in the following respects, but can still be integrated in PhotPack assemblies: n optical functionality n heights or directions of optical axes n component geometries and materials n manufacturing source or supplier n batch to batch variations For assemblies on a common base plate or flat micro-optical bench, there is only one horizontal optical plane. Optical components that already have an optical axis matched in height, and are of a geometry that can easily be picked via vacuum tools or piezo grippers like beam splitting prisms, can simply be moved around on the base plate. The orientation and fine alignment of the optical axis within the horizontal optical plane can be fixed during a sequential final assembly by means of a thin but adjustable epoxy gap between element base and base plate (typically in wedge form, with vertical distances from zero up to 50 µm). For optical components with a yet unmatched optical height, separate pedestals with the required height can be 2D-lasered from panel glass in the PhotPack approach. These pedestals can typically be doubled-up onto the optical component base, or be preattached by glueing in a passive alignment step on the base plate, using features such as laser-engraved marks, or discrete steps defined by other glass edges. Such pre-assembly steps can potentially be performed during final assembly in automated alignment machines used at IZM, taking advantage of powerful camera vision systems as well as very high accuracy relative positioning capabilities, which are described in more detail below. This preassembly just raises the component to the correct optical plane, they are still fine-alignable on the horizontal base plate as described before. For components that are very critical in the optical alignment, or which need defined degrees of freedom in the positioning, such as single optical mode focussing/collimating lenses or fiber collimators (which potentially are also polarization-sensitive), specific designs for holding elements have been created. These consist of several 2D-lasered glass parts, see Fig. 2, which are combined and interlinked to realize holding elements which are alignable in themselves, on top of being freely moveable versus the base plate for an overall horizontal alignment. Another advantage of PhotPack also has become obvious in the last para - graph: Many optical elements are not easily gripped, either since they are cylindrical and delicate, such as small lenses and optical fibers, or they are simply too small, either in all dimensions (< 1 mm for some micro lenses) or oblong with small gripping and mounting faces (as common for fast-axis-collimation cylinder lenses). For such components it can be highly advantageous to provide larger size rectangular frames for better automated gripping and mounting, while at the same time adapting to a common optical, as well as mounting plane, maintaining a horizontal alignability before a final fixing within the holding element or to the base plate, e. g. with UV-curable epoxy. Advantages of glass in microoptical assemblies Practically all technical and industrial glasses in sheet form may be used in PhotPack, with physical properties that can be selected to fulfill requirements that are either known from experience, or have been determined in optical or even thermo-mechanical simulations in the design phase. Since an active alignment in placing and glueing of holding elements is taking place anyhow, drawn Fig. 2 Left to right: Holding elements for fiber-assemblies made of laser structured flat glass parts in CAD. On the right: Raytraced holding element for an optical fiber ferrule (real element in Fig. 3) WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Optik&Photonik 3/

3 n high-rf-capable electrical traces Such features may be included along or transverse to the parallel planes defined by the glass sheets, with a potential for very high complexities in designs and functionalities. Fig. 3 Left: Parts of a base plate stack with mm 2 footprint, with built-in features used in the active-alignment process. Right: Holding element for embedding and aligning a polarization-sensitive fiber. float glass very often is a valid and inexpensive choice. If differences in thermal expansion have to be compensated this can inherently be done with the right sequence of glass slides and epoxies. When using UV-transparent glasses, epoxy glueing and curing can be performed on stacks, holding elements, or even full assemblies. Sandwich systems may also be pre-assembled by lamination or oven processes. Starting with large sheets, a panellevel manufacturing also becomes possible. Current limits at IZM are 600 mm 600 mm for laser processing with green and CO 2 -lasers with µm-accuracy, and 300 mm 300 mm for sub-µm automated alignment as further described below. To briefly sum up the advantages of glass used as mechanically supporting construction material in base plates and holding elements: n rather inexpensive base material n pre-defined thicknesses, typically 50 µm to 5 mm n available in large sizes (float glass or drawn sheets) n lamination before laser cutting and drilling possible n low CTE of 6 to 10 ppm/k, matching Kovar and CuMo metal parts (if additionally required) n dimensionally stable (+ can be tempered) n much higher transition temperatures than polymers Further functionalization of glass Apart from its superior optical and mechanical bulk qualities, and its overall and locally laser-selective structurability, glass can be further functionalized in additional processing steps. This can even be done in an alternating or intercombined manner, since PhotPack is performed by stacking or interleaving of laser-structured thin glass sheets. Additional features may be fabricated by direct laser writing or mask-assisted etching or development steps, preferably on large laminated or coated panels, before singulation into smaller parts. We for now have either realized or can think of integrating: n vacuum or pressure channels n capillary systems or reservoirs for liquids n thermal and DC-electrical conduction PhotPack demonstrators: Fiber coupling systems Some generic examples of assembly possibilities within PhotPack are given here, based on initial IZM designs, which currently are both being expanded for various application areas and specialized in customer-specific designs. The assemblies shown here do not include active components or light sources, but use external light sources in later operation as well as in the active alignment process. A precise mounting and optically efficient alignment of commercially available single-mode and polarization-sensitive optical components was possible. Until more details on performance and stability are published elsewhere, please contact the authors. The examples presented here visualize some of the concepts and elements in PhotPack: Optical elements are either directly movable on a base plate during active alignment, are adapted in their optical height to a common reference plane by pedestals, or are even made adjustable, by special design of the holding elements, compare Fig. 3 with a single fiber collimator holding element.. Fiber-optical end faces as well as optical axes can be finely aligned with six degrees of freedom (x, y, z and three angles), with a glueing of the constituting glass parts either in pre-assembly or different points in the final sequential Fig. 4 MDI-Schott LD600-H laser machine installation at IZM for the drilling, cutting and ablating of glass (left), with some green lasered sample structures shown (above, typical processing time per sample of a few minutes at most). 44 Optik&Photonik 3/ WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

4 Optical Components Fig. 5 From left to right: Ficontec AL1000 for sequential active alignment of optical components, with novel tool changing capability, part of the beam characterization system used in active alignment processes. alignment. How to procure the required 2D lasered glass parts is described in the following section. Production of glass parts from panels Non-contact machining of 2D glass panels by laser cutting and drilling has been firmly established in industry, e. g. in flat planel display and mobile phone production. To some extent 3Dstructuring of glass is possible by laser ablation, but this is still too slow for industrial application, as well as limited in aspect ratios and geometries. A state-ofthe-art industrial laser machine MDI- Schott LD600-H has been installed at IZM, combining pulsed green (532 nm wavelength) and CO 2 lasers (10.6 µm wavelength), along with optics and scanners to process flat panel glass (see Fig. 4 above). With pulsed green laser the following ablation processes are possible: n free form cutting of 2D contours n highly repeatable cuts, with µmresolution n shallow marking and rough grooving in 3D n drilling of holes with ca. 1:10 aspect ratio (down to 50 µm diameter) n max. panel size 600 mm 600 mm, typically < 5 mm thickness Contours in the panel glass are written layerwise, starting from the bottom of the glass, after focussing the green laser locally in the absorbtion and ablation site, while removed glass particles are falling down. Due to this processing method, a microscopical roughness is induced with an R a of typically 2 µm, which appears to be ideal for later glueing processes, when joining 2D parts as proposed in the PhotPack concept. The CO 2 -laser in the machine may be used for the thermally-assisted scribing and breaking of glass, yielding linear mirror cleaves on thin panels, with an optically smooth edge quality. It can also be used for laser pulse induced local remelting or evaporation processes, since most glasses are highly absorbing at 10.6 µm wavelengths. This way also holes may be structured, showing similarly small diameters and aspect ratios as obtained with the green laser drilling, but in approximately one tenth of the time. Please contact the authors for more information on this. Automated active alignment assembly In recent years, with advances in machine vision and motion systems for handling, trans lating and rotating small optical components, as well as the development of fairly easily programmable and reusable alignment process software, a class of universal automated optical pick and place assembly machines has been established. These initially were based on concepts used in pick and place machines for electrical components (chip-dies and surface mounted devices SMD), but had to be taken to the third dimension and sub-µm and few-arcsec positioning accuracies, since optical components generally are 3D objects with varying height in geometry and optical axes, and furthermore are highly alignment sensitive, especially in singlemode-operation systems with tuned optical path lengths or polarizations. IZM has been cooperating with machine suppliers and industrial customers in diverse projects, setting up assembly processes for photonic systems of various complexities and sizes. The installed machines commonly have 6 DOF (translations in x, y, z, and three angles of rotation) per actor/gripper of a component, with additional DOF on the assembly platform ( chuck with additional rotation axis and an optional x, y translation stage.) The assembly process is performed sequentially, using different vacuum tools and adjustable shape piezo grippers, see Fig. 5 above. In assembling and aligning optical systems, one can take advantage of the fact that the quality of the assembly can be fairly easily and directly be measured by analyzing optical coupling efficiencies during assembly. Either external or internal laser sources are operated actively, while the optical components to be assembled may still be moved into their optimal positions ( wet align in dispensed glue). Once optimal coupling efficiencies, pointing and collimation accuracies into optical fibers or into free space beams have been reached, the component being actively moved is fixed by UV-curing of highly enduring low-shrinkage epoxies. The most recently installed machine, a Ficontec AL1000, even has an automatic tool changer for more in-processflexibility. It offers two assembly stations 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Optik&Photonik 3/

5 and beam characterization units (near and far field, M 2 and optical power measurement). This machine was developed with partners Ficontec Services and Eagleyard Photonics, funded by Federal German BMBF in research project AutoFly, and is operated in clean room conditions. Another machine not shown here, a Ficontec AL500, is specialized on equipping optical single mode systems with footprints of up to 300 mm 300 mm. Evaluating hybrid and additive manufacturing systems To be even more flexible in the production of optical assemblies, alternative structuring approaches as well as alternative materials for hybrid system assemblies were investigated, with some results shown here. Alternative approaches to subtractive laser structuring processes for instance are additive processes, such as SLM (selective laser melting of powders) or FDM (fused deposition modeling with thermoplastic filaments). If a polymer material seems acceptable in the construction of an optical assembly, parts can directly be FDM- printed from 3D-CAD, in an already connected shape, formed continuously from a filament deposited in layers ( slices ) in an FDM-approach. Where optical fine alignment and transmission or mechanical and thermal stability is required, glass parts from the PhotPack approach described above may be used, to form hybrid assemblies. Two examples of 3D-printed designs are briefly described in the following, using an industrial-grade FDM-printer at IZM, a Fortus 360 mc (max. build volume of mm, with stated printing accuracies of +/ mm or +/ mm/mm). Typically, an object is sliced and printed per layer, starting with a closed outline of the slice, and filling this up before moving to the next layer. Depending on the material extrusion nozzle in the printer used at IZM, the minimum slice height is 130 µm, which also is the current layer-induced roughness. Due to the continuous printing technique, it is furthermore not possible to define sharp edges (as prevalent in the complementary glass-based PhotPack approach). It is possible, however, to dip-process the printed part and chemically smoothe out the layer structure, before further coating processes. To investigate the potential of this smoothing step, complex shape mirrors with up to twenty free form parameters (radius of curvature, conicity in different directions + higher mixed orders) were printed with a nozzle for 180 µm thick layers (taking eleven minutes per mirror as shown in Fig. 6), smoothed, and 3D-measured with CTscans to qualify the local smoothing and overall conserving of the intended mirror shape. For this first test, local smoothness as well as the deviation from the ideal shape was well controlled, and sufficient for the intended optical function at long wavelengths. For higher quality reflective beam forming systems, however, the expense of individally diamond-turned free form metall mirrors seems unavoidable. Optical systems requiring less complicated transmissive or more regular surfaces such as spherical and aspherical shapes, should also stick to designs using standard lenses made of glass or highly transmissive polymers, which are available with very high optical quality but which nevertheless have to be mounted and aligned Fig. 6 Top left: Hybrid test assembly of two assembled PhotPack glass base plates on a 3D-printed polymer chassis. Bottom left: Chemically smoothed free form mirrors (10 mm base width). Above: CT-scan analysis, showing the deviation from optically ideal structures (green range: +/ 50 µm). 46 Optik&Photonik 3/ WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

6 Optical Components Summary PhotPack by using 2D-discrete glass elements that can be designed and manufactured in almost arbitrary ways has proven its capability to combine and align very diverse optical elements, light sources, and detectors into robust 3D packages of various complexities. Using 2D-lasered glass slides as constituents comes at a certain cost assembly-wise, but we believe that the adaptability and inherent designability of machine-controllable degrees of freedom more than compensates this. Using glass as a base material offers wide design flexibilities at moderate material cost, with the potential to include more than just mechanical and optical functionality. Industrial grade automated laser processing and fine alignment machines nowadays are available, with the possibility to make use of cost effective panelwise fabrication of constituents or full assemblies. PhotPack can compensate fluctuations in optical and mechanical tolerances of optical components from external sources to a great degree. Up to now, the sequential processing has not proven to be a bottleneck or limitation. The laser drilling and cutting machine at IZM can, after loading of large panels, operate independently. Active optical alignment machines at IZM can either operate on panels of up to 300 mm 300 mm, and are equipped with automatic tool changing. For even higher volume production, they could be extended with automated part sorting and loading to minimize operator interaction. Most glass parts are currently being glued, but process development is under way to also include faster laser fusing and soldering processes. We observe that with every new assembly, be it a prototype or a small series assembly lot, new ideas and solutions emerge, while at the same time the production flow gets perfected and new functionalities are integrated. There is a high scalability and reusability of designs, parts, machine programming and processing, so that an economical production from single-count prototypes to series of several hundreds or even thousands s is plausible. DOI: /opph Authors Gunnar Böttger studied physics and received a doctoral degree from TU Hamburg-Harburg for a dissertation on photonic crystal optical circuitry. After being a post-doc at IPQ- KIT Karlsruhe he held industrial positions in Stuttgart and Berlin, before joining Fraunhofer IZM in 2012 as scientist focusing on photonic packaging. Stefan Seifert studied electrical engineering at TU Berlin. Between 2005 and 2011, he continued to work as a scientist and academic tutor, developing electronic devices. Since 2011, he has been an application engineer at Fraunhofer IZM for medical, automotive and consumer devices. Henning Schröder studied physics at the University of Magdeburg and got a Ph.D. from TU Berlin on anisotropic KOH etching of silicon for MoEMS. He currently is leading the group Optical Interconnection Technologies at Fraunhofer IZM, and active member of diverse scientific societies, networks and conference committees. Gunnar Böttger, Fraunhofer Institute for Reliability and Microintegration, Gustav-Meyer-Allee 25, D Berlin, Tel.: , Fax: -271; gunnar.boettger@izm.fraunhofer.de 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Optik&Photonik 3/