page 1
page 2
page 3
page 4
page 5
page 6
page 7
page 8
page 9
page 10
page 11
page 12
page 13
page 14
page 15
page 16
page 17
page 18
page 19
page 20
page 21
page 22
page 23
page 24
page 25
page 26
page 27
page 28
page 29
page 30
page 31
page 32
page 33
page 34
page 35
page 36
page 37
page 38
page 39
page 40
page 41
page 42
page 43
page 44

PFIB INMICROELECTRONICSanalysis of the structures difficult or evenimpossible. Figure 2 shows typical curtaining effects onthe silicon substrate as well as the TSV itself,caused by milling through the overlying roughpoly crystalline metal film. These curtainingeffects can be effectively suppressed by rock-ing the sample during the FIB milling process.Milling in a sequence of alternating incidenceangles creates a clean cross section free of cur-taining artifacts without the need for time-consuming low current polish steps.EXAMPLES OF APPLICATIONS OFPLASMA FOCUSED ION BEAM Through Silicon ViasTSVs are themselves subject to a number ofeffects that can result in defects and failures.For example, the large differences in thermalexpansion between copper via fill and the sur-rounding silicon substrate can cause crackingwithin the copper and delamination from thevia sidewall during thermal processing. 'Key-holing' results from incomplete filling of vias(Figure 3).Solid Liquid Interdiffusion BondingOne of the most difficult issues to address isthe behavior of bonds between chips duringsubsequent processes (Figure 4). For example,it is critical that a bond between the first chipsin the stack not be disturbed by the subse-quent bonding of an additional chip. Solid-liquid-interdiffusion (SLID) [4] is a uniquedirect metal bonding technology that avoidsremelting of existing bonds during the forma-tion of new bonds by using high melting inter-metallic phases. During bond formation, solidmetal diffuses into the liquid phase of a lowermelting metal resulting in high melting pointfinal phase that remains solid during subse-quent bond forming processes.Anisotropic Conductive AdhesivesAnisotropic conductive adhesives (ACA) can beused [5] to bond wafers together physicallyand electrically using an organic bonding com-pound (benzocyclobutene, BCB) filled with 4-?m sized metal covered polymer spheres(MPS). The BCB assures mechanical strengthwhereas the MPS provide the required electri-cal conductivity at interconnection points. Theconcentration of MPS must be high enough toensure good electrical contact betweenopposed pads and at the same time lowenough to guarantee electrical insulationwhere pads are not present. To study the bonding in detail, samples werecleaved, then milled with the plasma-FIB toreveal the bonding region and finallyinspected with plasma-FIB imaging. Theplasma-FIB milling speed makes it possible toprepare the sample (~200 ??50 ??600 ?m3material removed) within 30 minutes. Themetal layer covering the polymer spherescould be observed at the bonding interfacewith sufficient resolution to estimate both thelocal MPS density and their compression statebetween the bond pad metal layers. In Figure5 the bonding process is illustrated in the topFigure 4: The void between these pads is the result of an incomplete SLID bonding process. The various intermetallic phases are clearly visible above, below andto the right of the void. MICROSCOPY AND ANALYSISNOVEMBER 201111Figure 3: (a, b) Differing thermal expansionbetween copper via fill and silicon substrate caused delamination shownin this via before (a) and after (b)annealing. (c) Keyholing occurred when this viawas not filled completely with tung-sten.caCu Cu3SnCu6Sn5FIB debrisb

MICROSCOPY AND ANALYSISNOVEMBER 122011images and the bottom plasma-FIB imagesshow details of the bonding interface andcompressed spheres. 3D Test ChipThe 3D integrated reliability test chip shown inFigure 6 is a 3-level-stack with a modular lay-out designed to permit evaluation of assemblyprocesses between two initial layers and, sub-sequently, the effects of adding a third layer[6]. The PFIB can mill a cross section throughthe entire three layer stack showing criticaldetails of both upper and lower bondingregions and the complete TSV through themiddle layer.CONCLUSIONSBy combining high-speed milling and deposi-tion with precise control and high qualityimaging, the plasma focused ion beam pro-vides critically needed physical analysis for TSVand bonding processes that are essential tocurrent 3D integration schemes. At high beamcurrents, cross-sections with dimensions ofhundreds of micrometers can be completed inless than an hour, fast enough to provideeffective feedback on process performance. Atlow beam currents, the same system delivershigh resolution imaging for accurate structuralanalysis. The PFIB provides an effective, practical toolfor a variety of 3D integration applications,including failure analysis of bumps, wirebonds, TSVs, and stacked die; site specificremoval of package and other materials toenable failure analysis and fault isolation onburied die; circuit and package modificationsto test design changes without repeating thefabrication process or creating new masks;process monitoring and development at thepackage level; and defect analysis of packagedparts and MEMS devices.REFERENCES1.Smith, N. S., Skoczylas, W. P., Kellogg, S.M., Kinion, D.E.,Tesch, P.P., Sutherland, O., Aanesland, A., Boswell, R.W. HighBrightness Inductively Coupled Plasma Source for HighCurrent Focused Ion Beam Applications. J. Vac. Sci. Technol.B24(6):2902-2906, 2006.2.Kellogg, M., Schampers, R., Zhang, S.Y., Graupera, A.A.,Miller, T., Laur, W.D., Dirriwachter, A.B. High ThroughputSample Preparation and Analysis using an InductivelyCoupled Plasma (ICP) Focused Ion Beam Source. Microsc.Microanal. 16(Suppl 2):222-223, 2010.3.Kwakman, L., Franz, G., Taklo, M. M. V., Klumpp, A., Ramm,P. Characterization and Failure Analysis of 3D IntegratedSystems using a novel plasma-FIB system. Proc. InternationalConference on Frontiers of Characterization and Metrologyfor Nanoelectronics, Grenoble, France, 2011.4.Ramm, P. Method of making a three-dimensional integratedcircuit. US Patent 5,563,084; P. Ramm, A. Klumpp. Methodof vertically integrating microelectronic components. USPatent 6,548,391.5.Taklo, M. et al. Anisotropic Conductive Adhesive for Wafer-to-Wafer Bonding, Proceedings of 7th Intl Conference andExhibition on Device Packaging, March 2011.6.Ramm, P., Klumpp, A., Franz, G., Kwakman, L. FailureAnalysis and Reliability of 3D Integrated Systems. Proc.IMAPS Device Packaging Conf., Scottsdale, Arizona, 2011.©2011 John Wiley & Sons, LtdFigure 6: The high milling speed of PFIB permits cross-sections through the full three layer stack of the test chip, revealing both upper and lower bonding regionsand the entire TSV.Figure 5: Anisotropic conductive adhesives provide mechanical bonding and electrical conductivity. (a,b) Images show metal coated spheres before mixing withBCB (a), and a schematic of how TSVs can be electrically connected to pads on another wafer using BCB filled with such spheres (b). (c-f) The lower four images are a clockwise sequence of increasing magnification with the compressed metal coated spheres clearly visible in the twobottom images. fedabc