TEM Specimen Preparation by Focused Ion Beam Sputtering - Optimisation of the Process machining of T EM lamella with Focused Ion Beam sputtering B. Köhler, G. Irmer, L. Bishoff, J.Teichert advantages : high positioning accuracy little restriction on samle material stress free-milling observation of progress (SEM or SIM) - Außens telle E ADQ Dres den - Forschungszentrum Rossendorf e.v. Institut für Ionenstrahlphysik und Materialforschung Postfach 51 119, D-1314 Dresden ion beam TEM-observation direction TEM-observation direction
mas hining of T E M - lamella LRQV VSXWWHU UHPRYDO 7( ODPHOOD E xample: lamella in a fracture mechanique s pecimen ion beam TEM-observation direction Inclusion, which was the reason for the fracture leveling inclusion T E M images
T E M images T E M images 2 µm 2 nm machining of T E M lamella with F ocus ed Ion B eam s puttering E xample for bending: advantages: high pos itioning accuracy little res triction on sample material s tress free-milling observation of progress (S E M or S IM images ) pos s ible problems /dis advantages : amorphis ation contamination of the s ample by Ga (at leas t in the machined area) bending of the lamella due to s tres s releas e aim varied parameter avoid contamination with Ga s ubs titute s ource Ga S i,... reduce the thicknes s of the amorphis ation layer angle of incidence energy ion mas s influence of the ion angle can the penetration depth ( ==> the thicknes s of the of the dis turbed layer) reduced? Í s peed of machining (s putter efficiency) s ort of ions, energy, current reduce bending of lamella additional cuts for s tres s relaxation LRQ EHDP GDPDJHG UDQJH
influence of the ion angle can the penetration depth ( ==> the thickness of the of the disturbed layer) reduced? Monte Carlo s imulation (S RIM J. F. Ziegler, IBM) depth / A 5 45 4 35 3 25 2 15 1 5 Si (3 kev) into Si cos(α) * depth () is a good fit only for small α LRQÃEHDP GDPDJHG UDQJH Simulation: projected range/ A 5 4 3 2 1 Au 3kV Au 1kV Ga 1kV Ga 3kV Si 1kV Au 8kV Si 8kV Si 3kV 3 6 9 incident angele -5 2 4 6 8 α simulation results require experimental verification! R aman Spectroscopy triple monochromator CCD analysator spatial filter beamsplitter T64 Jobin-Yvon Micro-Raman- Spectrometer sample cryostate mirror y x microskope objective table mirror cryostate sample polarisator macro chamber microscope laser Raman-measurement of the thickness of the amorphous layer c-s i a-s i d c-s i I c1 I a I c2 α c α a α c Cts/s 8 6 4 2 8 4 2 4 6 Raman shift (cm -1 ) x 2 x 2 3 portion of the amorphous layer 2 Si wafer, sputtered with Ga ions (3 kev, 3 ) Cts/s 1 8 4 8 6 4 2 sputtered area c-si 2 4 6 Raman shift (cm -1 ) x 2 x 2
I c1 α c d I a I c2 measured intensities : α a I c1 : crystalline material only I a : from the amorphous layer αc I c2 : from crystalline material below the amorphous layer S i Wafer with machined areas in the light micros cope measurement points: 1: 6 2: 3: 3 Ia βα a c 2 α = e a d 1 I α c, αa - absorption cross section c2 αβ a c βc, βa - Raman scattering cross section I (1 r ) e I r,r c2 = a 2 a d c1 (1 r c) α c a - reflection coefficients Solving for the thickness d: 1 rc Ia 1 r I 1 r I d ln 1 r I a c2 a c1 = βα a c I c1 c c2 1 βc Ic2 βα 1 r I α a = Ia I a β a 1 rc Ic2 c c1 c c2 1 c-s i Comparison of first results: Ga penetration depth by S RIM s imulation <-> amorphisation thickness by Raman measurements E nergy: 3 kev angle of range (SRIM) measured amorphisation layer incidence thickness d 6.9 (extrapolated) 7. nm 3 7.3 nm 7.5 nm 6 7.7 nm 7.2 nm 9 8.2 nm -- for use of other beam ions than Ga alloy s ources and an ion column with a mass filter must be available principle of the FIB 44 it includes a mass filter source control blanking control scan generator amplifier imaging system Ion source extractor condenser blanking plates Wien filter (ExB) (selection of ions by mass) X,Y- deflection plates lens secundary electron detector precursor gas injector Parameters of the FIB 44 system dual beam chamber 4 stage eucentric (6 axis ) imaging detection of s econdar y electrons F IB : CANION 31Z ions Ga, Au, S i, Ge mass selection m/ m=35 energy...3 kev current 1 pa... 2 na resolution 1 nm precurs or gas es W, F -, Cl - SEM: LEO 44 filament W, Lab6 EDX Röntec s pecial access to software interface proxy writer
developed s ource module image taken with an Au Ge Si beam Crossover mode I = 1 na Summary: lowering the energy reduces the penetration depth (thinner contamination + amorphisation layer), reducing the angle between ion beam and surface to small values has negligible effect to the penetration depth varying the ion gives an additional free parameter for optimisation (high sputter yield for heavy ions, e.g. Au, small penetration depth) working with a S i beam s hould avoid the contamination problem in S i wafer applications an AuGeS i alloy source has been installed successfully at the FIB 44 in the mixed ion version the alloy source works satisfactory What remains to do / next s teps : detailed tes t of the alloy AuGeS i source experimental (FIB + Raman) verification of the SRIM results