FIB and the sample
FIB and the sample Hugo Bender Chris Drijbooms, Patricia Van Marcke, Paola Favia, Olivier Richard, Jef Geypen, Koen Marrant, Eveline Verleysen Leuven, Belgium 4. FIB-Workshop Focused Ion Beams in Research, Science and Technology Halle, 29./30. Juni 2009 Outline • Introduction : FIB and application • Ion beam – image quality – curtaining – redeposition – discharges • Ion beam interactions with – Si – metals – Al – Ni • Conclusions Hugo Bender - PT/MCA/SA © imec 2009 2 FIB/SEM configuration • “Dual beam” • “CrossBeam” • “MultiBeam” Figures ©FEI Hugo Bender - PT/MCA/SA © imec 2009 3 FIB applications – Cross-section imaging contacts still buried Cu gate Si small structures (10-20 nm dimensions) © Anabela Veloso Hugo Bender - PT/MCA/SA © imec 2009 4 FIB applications – Cross-section imaging huge structures (50 µm dimensions) Si Cu 4 to 10 h milling ! Hugo Bender - PT/MCA/SA © imec 2009 5 FIB applications – TEM preparation 3 mm Cu grid metal needle metal needle 2 : lift-out 3 : attach to Cu grid 1 : mill “chunk” free 4 : attached to Cu grid metal needle 5 : final thinning Cu grid 3 mm Cu grid Hugo Bender - PT/MCA/SA © imec 2009 6 FIB applications – TEM preparation AFM cantilever Pt weld Cu grid cut Pt weld omniprobe omniprobe diamond tip is on the backside Cu grid TEM viewing direction Hugo Bender - PT/MCA/SA © imec 2009 7 FIB applications - Other • Atom probe tips • Back-contacting for SSRM • Marking • Device modification • Micro-machining • Biological © Sebastian Koelling Hugo Bender - PT/MCA/SA © imec 2009 8 Interactions with the substrate 40-30-5-2 keV Hugo Bender - PT/MCA/SA © imec 2009 9 30 keV 0.92nA Ion beam image quality 5 keV 1.0 nA 2 keV 0.77nA particle on the surface Hugo Bender - PT/MCA/SA © imec 2009 10 Ion beam image quality sample edges before “clean” milling 30 kV 5 kV 2 kV sample edges after “clean” milling more rounding on the milled edge Hugo Bender - PT/MCA/SA © imec 2009 12 Redeposition 30 keV staircase 5 keV clean ion-Pt e-Pt e-Pt “no” redeposited material on the surface ! redeposited material 30 keV a-Si Hugo Bender - PT/MCA/SA © imec 2009 13 Redeposition FIB staircase SEM 10 µm BCB via etched from the backside through 50 µm Si redeposited material FIB major redeposition occurs strongest on top side of open structures Hugo Bender - PT/MCA/SA © imec 2009 14 Redeposition during Pt deposition 10 µm redeposition around 2x2x0.5 µm Pt pads Pt: 30 keV 100 pA Pt: 5 keV 75 pA Pt deposition = competition deposition and milling ! © Jay Mody Hugo Bender - PT/MCA/SA © imec 2009 15 Curtaining 25 µm Cu TSV trenches with barrier/Cu seed layer only • related to beam tails : worse at lower keV • induced by fast/slower milling materials or topography • can be “avoided” for TEM preparation : backside milling Hugo Bender - PT/MCA/SA © imec 2009 16 Curtaining : low-k/Cu Cu SiOC k 7 4 11 N2/O2 lowest in situ O2 medium N2/O2 highest EFTEM AFM low k/Cu relative thickness % step from step below the lines Cu/lowk in oxide in Si nm nm nm 40 7.6 4.9 3.4 60 5.2 4.0 2.6 80 6.4 4.4 3.5 FIB-induced steps are similar, i.e. low-k density varies due to processing conditions, scales with k-values S. Hens et al, Phys. Conf. Ser. 169, 415-418 (2001). Hugo Bender - PT/MCA/SA © imec 2009 17 Curtaining : 50µm open TSV – TEM preparation • Deep structures : mounting orthogonal • Very high aspect ratio structures are difficult to fill with Pt H-bar TEM specimen : SEM TEM 64 Cu 12 TaN Hugo Bender - PT/MCA/SA © imec 2009 18 Curtaining : 50µm open TSV – TEM preparation cleaved standard lift-out wafer surface curtaining problem avoided 50 um chunks ready for lift-out specimen landed on top of the TEM grid Cu grid Hugo Bender - PT/MCA/SA © imec 2009 19 Discharge damage in Kelvin structure top view tilted cross-section through damage Hugo Bender - PT/MCA/SA © imec 2009 20 Breakdown charge V= QFIB Cox QFIB= (E× t ox ) × (εo × εr × S/tox ) = E × εo × εr × S • Kelvin structure : area S ~ 20000 µm2 • Breakdown field for CVD oxide ~ 10 MV/cm • breakdown charge ~ 0.7 nC imaging 1-4 pA : 700-175 s crater with 2700 pA : 0.26 s i.e contacting in advance necessary ! Hugo Bender - PT/MCA/SA © imec 2009 21 Ion beam interactions : Si • many materials are completely amorphized by the ion beam – Si, Ge, III-V, … – silicides – many oxides FIB • indication for amorphisation : absence of channeling contrast Hugo Bender - PT/MCA/SA © imec 2009 22 Damage layer in Si 10 vs 30 keV : normal incidence Pt 10 keV 30 keV Hugo Bender - PT/MCA/SA © imec 2009 23 Interaction layer 30 keV Pt depostion Auger spectroscopy depth profile wedge FIB prepared Hugo Bender - PT/MCA/SA © imec 2009 24 Surface protective layers • properties – > 150 nm – not reacting with the top layer – contrast in TEM with top layer, preferably amorphous, light elements – not planarising the topography – stress free • options – wafer process line : a-Si, poly-Si, stress-free nitride, … – low-T CVD glass – sputtered glass – sputtered/evaporated Al or Ni – e-beam Pt or W Hugo Bender - PT/MCA/SA © imec 2009 26 Au and glue protective layers Au Au on glue Hugo Bender - PT/MCA/SA © imec 2009 27 e-Pt capping e-Pt • shrinkage of the low-k and collapse of the barrier low-k • Pt diffusion in a-Si SiO2 e-Pt e-Pt Cu low-k TaN Hugo Bender - PT/MCA/SA © imec 2009 28 Si sidewall damage and slope “simulated” specimen in-situ ion-Pt covered 70 pA 150 pA 150 pA 1000 pA +/-0.4º Hugo Bender - PT/MCA/SA © imec 2009 29 Si sidewall damage : 30 keV Ga 60-65 nm 20-23 nm Hugo Bender - PT/MCA/SA © imec 2009 30 Reducing sidewall damage : 5 keV sputtering top 6 µm deep 150 pA +/-0.4º 150 pA +/-0.4º 5 keV Ga 15º Hugo Bender - PT/MCA/SA © imec 2009 31 Reducing sidewall damage : XeF2 etching 150 pA 0º tilt XeF2 30 s - no beam amorphous layer etched XeF2 : very efficient but useless due to the high roughness also creates steep topography at interfaces Hugo Bender - PT/MCA/SA © imec 2009 32 Reducing sidewall damage : low-keV Ar milling Ar thinning on both sides after 30 keV Ga thinning : - Ar 1 keV for 6 min., angle of incidence 75º - Ar final thinning 300 eV for 15 min. - reduction of amorphisation to 3-4 nm Hugo Bender - PT/MCA/SA © imec 2009 34 Ion beam interactions : metals • channeling contrast occurs in all freshly milled metal, e.g. Al, Cu, Ni, W, Au, TiN, … indicating that full amorphisation does not occur • no channeling in TiAl3 Hugo Bender - PT/MCA/SA © imec 2009 36 Ion beam interactions : metals A comparison of EDS microanalysis in FIB-prepared and electropolished TEM thin foils, C.R. Hutchinson, R.E. Hackenberg, G.J. Shiflet, Ultramicroscopy 94, 37–48 (2003) α Cu -β Cu4Ti comparison of electropolished and FIB prepared specimens of Cu / Cu4Ti : • large increase in dislocation density attributed to the collapse and coalescence of point defects created during milling + high defect mobilities in metals • no Ga is detected in the EDS spectra Hugo Bender - PT/MCA/SA © imec 2009 37 Ion beam interactions : metals 30 keV ion-Pt deposition original surface Pt Pt Pt Al Hugo Bender - PT/MCA/SA © imec 2009 Cu 38 Interaction layer ion-Pt on TiN TEM HAADF-STEM ion-Pt : complimentary Pt+Ga vs C Pt-rich C-rich Pt-Ga-TiN Ga tail wedge FIB prepared measurements Ann De Veirman, NXP Hugo Bender - PT/MCA/SA © imec 2009 39 Ion beam interactions : Al on Si • DC/magnetron sputtered Al / native oxide / Si (200 mm wafer) • idem covered with 150 nm low-T CVD SiO2 layer • idem with TiN/Ti barrier, after anneal : TiN/TiAl3/Al • in-situ lift-out in a dual-beam FIB/SEM : – standard specimens finished 30 keV Ga – “cross-specimen” finished 30 / 5 / 2 keV Ga Hugo Bender - PT/MCA/SA © imec 2009 40 Al / thin oxide / Si : TEM - HREM 30 keV Ga Al lattice is continuous in the dark layer Al Al Al dark contrast layer in the Al near Al/SiO2/Si interface Hugo Bender - PT/MCA/SA © imec 2009 41 Al / thin oxide / Si : HAADF-STEM 30 keV Ga HAADF-STEM Al bright contrast layer : at Al-Al grain boundaries in the Al near Al/SiO2/Si interface Si 50 nm P. Favia, EMC2008 Hugo Bender - PT/MCA/SA © imec 2009 42 Al / thin oxide / Si : HAADF-STEM – EDS/EELS 30 keV Ga EDS/EELS : - accumulation of Ga in Al near the interface - width of the Ga profile is much larger than on the images and depends on the sense of the linescan Al Si EDS – EELS (O) 100 50 nm Si O O 100 Counts (a.u.) Counts (a.u.) 80 Al Ga Al 60 40 Ga Si 50 20 0.0 0.0 5 10 15 Position (nm) P. Favia, EMC2008 20 0.0 5 10 15 20 Position (nm) Hugo Bender - PT/MCA/SA © imec 2009 43 Al-Al grain boundary : HAADF-STEM – EDS ion-Pt EDS 80 30 keV Ga 60 CVD-SiO2 Counts Al 40 Al Ga 20 0.0 100 nm accumulation of Ga at the Al grain boundaries 10 20 Position (nm) 30 and EDS – EELS (O) 60 ion-Pt 40 Al at the CVDSiO2/Al Counts (a.u.) O CVD-SiO2 40 Si 20 Ga Al 200 nm 0.0 10 20 Position (nm) P. Favia, EMC2008 30 40 Hugo Bender - PT/MCA/SA © imec 2009 44 TiN / TiAl3 /Al : HAADF-STEM – EDS 30 keV Ga 100 SiGe TiN TiAl3 Al 80 SiGe Ge Al TiN TiAl3 Counts 60 Ti 40 Ga Al 20 Si TiAl3 50 500 500 nm nm 500 500 nm nm nm 500 100 150 Position (nm) accumulation of Ga at TiN/TiAl3 and TiAl3/Al interfaces P. Favia, EMC2008 Hugo Bender - PT/MCA/SA © imec 2009 45 Al / ion-Pt – trench sidewall : TEM / HAADF-STEM staircase + clean without or with extra tilt “immediately” ion-Pt or e-Pt deposition 30 keV Ga HAADF-STEM (no tilt during clean : ~ 2º slope) TEM 16 nm gray contrast ion-Pt rounded at the a-Si Al Al ion-Pt Si 43 nm Si 20 nm 22 nm a-Si Hugo Bender - PT/MCA/SA © imec 2009 47 Al / ion-Pt – trench sidewall : EDS/EELS 150 30 keV Ga EDS – EELS (O) O ion-Pt Pt 100 Al Si Counts (a.u.) Al 50 Ga 20 nm 0.0 10 20 30 40 Position (nm) Al / Pt-Al / Al-O / Pt Ga profile in the bright region Hugo Bender - PT/MCA/SA © imec 2009 48 Al / ion-Pt – trench sidewall 30 vs 5 keV 30 keV 5 keV gray contrast ion-Pt rounded at the a-Si Al 43 nm Si 22 nm 6 nm a-Si Hugo Bender - PT/MCA/SA © imec 2009 49 Ion beam interactions : Ni on Si • evaporated Ni on Si • silicide formed at the interface during the deposition • specimens – chunk / plan parallel specimen : finished with 30 keV Ga and tilted to compensate the slope – chunk / needle specimen as for atom probe : finished with 2 keV Ga, no tilt possible Hugo Bender - PT/MCA/SA © imec 2009 50 Ni on Si – 30 keV FIB lift-out - HAADF-STEM increased contrast/brightness ion-Pt Ni Ni 95 nm Ni Si Si Ni-silicide layer 50 50 nm nm extra layer : ~10%Ni/90%Si (EDS) Si 10 nm nm 10 E. Verleysen / S. Koellilng, tbp 55 nm nm Hugo Bender - PT/MCA/SA © imec 2009 51 Ni on Si – needle finished 2 keV - TEM slope ~11º (SiGe multilayer) E. Verleysen / S. Koellilng, tbp Hugo Bender - PT/MCA/SA © imec 2009 52 Ni on Si – needle finished 2 keV - TEM Ni amorphous Ni-Si crystalline overhang 4 nm a-Si Si E. Verleysen / S. Koellilng, tbp Hugo Bender - PT/MCA/SA © imec 2009 53 Ni on Si – needle finished 2 keV – HAADF-STEM Ni-rich overhang Ni-silicide 3.6 nm 8.7 nm 50 50 nm nm 10% Ni / 90% Si (EDS) E. Verleysen / S. Koellilng, tbp Hugo Bender - PT/MCA/SA © imec 2009 10 10 nm nm 10 10 nm nm 54 Ni on Si – sloped Ni thickness e-Pt deposited afterwards Ga beam 70 nm Ni-Si mixing under the Ni of the sloped Ni edge E. Verleysen tbp Hugo Bender - PT/MCA/SA © imec 2009 55 Ni reaction • Ni reacts with the amorphized Si, forming a Nisilicide layer on the outsides of the TEM specimen (a ring in case of needle sample) • the “10% Ni” layer thickness : – 30 kV 3.5 nm – 2 kV 8.7 nm thickness difference likely related to different slope : “0º” for the chunk vs “11º” for the needle • under thin Ni also Ni-Si forms due to the Ga beam • compare : – Si-Cr reaction with 30 keV Ga in Si/Cr stack : Barna et al., J. Appl. Phys. 102, 053513 (2007) Hugo Bender - PT/MCA/SA © imec 2009 56 Conclusions - outlook • the ion beam is always interacting with your sample • needs – better low keV image quality – faster milling systems (plasma-FIB, higher energy, higher currents) Hugo Bender - PT/MCA/SA © imec 2009 57 Hugo Bender - PT/MCA/SA © imec 2009 58 EFUG2009 : 5 October 2009, Arcachon http://www.imec.be/efug/ Hugo Bender - PT/MCA/SA © imec 2009 59
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