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solutions for the fabrication of high value-added heterogeneous components and sys-tems including memories, logic, sensors, actu-ators and wireless communications. Partici-pants in the program include materialproviders, laboratories, research centers andmanufacturers of equipment, componentsand systems.The project is structured around five themes:·Methodology and evaluation tools to inte-grate elementary components in 3D systems.·3D technologies and integration processesonto materials and non-silicon substrates.·3D technologies and integration processesonto silicon, using processes closely derivingfrom microelectronics.·Reliability methodologies and analysis forintegrated 3D systems.·Performance evaluation and equipment val-idation for volume production equipment andgeneric manufacturing.SINTEF, The Fraunhofer Institute and FEI areparticipants in JEMSiP_3D, and the work pre-sented here was funded in part by the project.3D INTEGRATIONWhile the performance and productivity ofmicroelectronics have increased continuouslyover more than four decades due to the enor-mous advances in lithography and device tech-nology, it has now become questionable ifadvances in these areas alone will be able toBIOGRAPHYMaaike M. V. Takloreceived her PhD in phys-ical electronics from theUniversity of Oslo in 2002for her thesis entitled'Wafer bonding forMEMS'. From 1998 until2010 she was employed at the Departmentof Microsystems and Nanotechnologywithin SINTEF ICT in Norway where sheworked on MEMS processing and wasresponsible for their wafer level bondingactivities. Maaike is now a senior scientist atSINTEF ICT at the Department of Instrumen-tation and is the group leader for 'AdvancedPackaging and Interconnects'.ABSTRACT3D integration schemes connect stackedintegrated circuits using through silicon vias(TSV) and special bonding techniques. Phys-ical characterization of these TSVs andbonds is essential, but their relatively largesize (tens or hundreds of micrometers)requires prohibitively long milling times inthe conventional focused ion beam (FIB) sys-tems typically used for this work. A newplasma-based FIB system can remove mater-ial more than 20 times faster, providing thespeed and precision required to ensurerobust processes and reliable products.KEYWORDSfocused ion beam, scanning electronmicroscopy, plasma ion source, ion beammilling, 3D integration, through silicon viasACKNOWLEDGEMENTSA part of this work has been performed inthe project JEMSiP_3D, which is funded bythe public authorities in France, Germany,Hungary, The Netherlands, Norway andSweden, as well as by the ENIAC JointUndertaking.AUTHOR DETAILSDr Maaike M. Visser Taklo, SINTEF ICT, Department of Instrumentation,PO Box 124 Blindern, N-0314 Oslo, NorwayTel: +47 2206 7300Email:MaaikeMargrete.VisserTaklo@sintef.nowww.sintef.noMicroscopy and Analysis25(7):9-12 (EU), 2011PFIB INMICROELECTRONICSINTRODUCTION3D integration schemes, which stack inte-grated circuits and other microelectronic orMEMS devices and interconnect them usingthrough silicon vias (TSV), are likely to be thenext revolution in electronic fabrication. Theycan be used to continue the increases in speedand density of microelectronic systemsdescribed by Moore's Law (More Moore), butthey may offer even greater benefits whenused to connect devices of different technolo-gies (More than Moore), packing more perfor-mance and functionality into smaller volumes. In either case, the ability to physically char-acterize the TSVs and mechanical bonds usedin 3D integration is essential for developingrobust manufacturing processes and fabricat-ing reliable products. Focused ion beam (FIB)systems have long provided physical analysisin the manufacture of integrated circuits, butconventional FIB cannot remove material fastenough to analyze these relatively large struc-tures used in 3D integration. The launch of anew plasma-based FIB system now providesthe speed and precision needed to developand deploy these exciting new technologies.JEMSiP_3DThe Joint Equipment and Materials for System-in-Package and 3D Integration (JEMSiP_ 3D) isa project undertaken by a consortium of Euro-pean manufacturers to validate technologicalBonding and TSV in 3D IC Integration:Physical Analysis with a Plasma FIBMaaike M. V. Taklo,1Armin Klumpp,2Peter Ramm,2Laurens Kwakman3and German Franz31. SINTEF, Oslo, Norway. 2. Fraunhofer EMFT, Munich, Germany. 3. FEI Company, Eindhoven, The NetherlandsFigure 1a: Schematic of a xenon plasma focused ion beam (PFIB) system. A PFIB uses an inductively coupled plasma to deliver high beam current. The source islarger than a liquid metal ion source (LIMS), but delivers a more collimated beam, enabling better beam spot performance at high beam currents.MICROSCOPY AND ANALYSISNOVEMBER 20119a

overcome the predicted performance and costproblems of future IC fabrication. The ITRSroadmap predicts 3D integration as a key tech-nology to overcome this so-called 'wiring crisis'and the solution will most likely be based onTSV technology. The most promising 3D integration schemescurrently under consideration involve the ver-tical stacking of integrated circuits and otherdevices. These schemes vary in their details butall must solve two central problems: how tobond the integrated layers together and howto create electrical connections among them.Bonding and TSV technologies each have theirown unique set of considerations which oftencenter around how the structure will hold upduring subsequent processing, such as theaddition of another layer:·Will the stresses induced by additional ther-mal processing cause debonding or shifting ofthe existing bonds? ·Will the stress and strain cause cracks ordelamination in the TSVs? ·What are the best materials and processesto use to minimize these negative effects?PLASMA FOCUSED ION BEAMFIB systems, which use an ion beam to cut andimage cross sections through subsurface struc-tures with nanoscale precision and imagingresolution, have long been a mainstay of phys-ical analysis for integrated circuits. Althoughthe structures used in 3D integration can beexpected to decrease in size as the technolo-gies evolve, they are much larger than thedimensions of the transistors and intercon-nects used in current integrated circuits, andthe cutting speed of FIBs designed for ICs isgenerally inadequate for TSVs and bondingstructures. A typical 10 ?m ??10 ?m IC cross-section requires the removal of 1000 ?m3ofmaterial and takes a few minutes. A 100 ?m ??100 ?m TSV cross-section requires the removalof 1,000,000 ?m3of material and would takemost of a day with conventional FIB.The Vion PFIB system (FEI Company, Hills-boro, Oregon, USA) uses an inductively cou-pled plasma source [1-3] (Figure 1) to providematerial removal rates 20??faster than con-ventional FIBs that use liquid metal ion sources(LMIS). A LMIS is essentially a point source 50nm in diameter with a low angular intensity.The Vion system's plasma source is larger, 15?m, but has a much higher angular intensity.Because of its small virtual size, the LMIS is easyto focus into a small spot at low beam currents,but at beam currents above 10 nA sphericalaberration effects severely degrade perfor-mance. The plasma source can deliver currentsin excess of a ?A (>20??greater than a typicalLMIS based system) while still maintaining awell focused beam. Since material removalrates are primarily a function of beam current,the PFIB has an advantage of 20??or moreover conventional FIB at high currents, whilestill preserving excellent milling precision andimaging resolution at low beam currents.The xenon ion beam emitted by the plasmasource has high sputtering yield, high bright-ness and low energy spread. In addition, byintroducing various gases, the PFIB can selec-tively etch specific materials or deposit pat-terned conductors and insulators (similar toconventional FIB systems). The plasma sourcealso offers the potential to use different ionspecies to enhance performance in specificapplications.CURTAININGThe difference in FIB milling rates of the vari-ous materials present in a device (Cu, Si, Sn,dielectrics, polyimides and mold compounds)can cause 'curtaining' when milling cross-sec-tions. This milling artifact can make detailedFigure 2: Curtaining artifacts (upper left), caused by variations in milling rate for different materials, can be effectively suppressed (right) by rocking the sampleto mill in a sequence of alternating angles (lower left).MICROSCOPY AND ANALYSISNOVEMBER 102011Figure 1b: The PFIB maintains excellent spot size performance over a broad range of beam currents. Figure 1c: At high beam currents the PFIB can remove material twenty times faster than a liquid metal ion source.bc