FOCUSED ION BEAM TECHNIQUES FOR

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1 FOCUSED ION BEAM TECHNIQUES FOR RESIDUAL STRESS ANALYSIS AT THE MICRON-SCALE Marco Sebastiani University of Rome «Roma TRE» Engineering Department Materials Science and Technology Group 7th Annual FIB SEM Workshop Thursday, February 27th, 2014 Kossiakoff Center Johns Hopkins Applied Physics Laboratory Laurel, MD

2 RESIDUAL STRESS ANALYSIS ON A MICRON-SCALE Thin coatings and MEMS Intra-grain stress analysis in polycristalline alloys and TBCs 2

3 GAP OF KNOWLEDGE Research needs Many advanced materials and devices require stress measurement with sub-micrometer 3D spatial resolution Existing high resolution methods (i.e. using synchrotron XRD, EBSD) are very expensive, time consuming and not suitable for amorphous materials Production needs New developed procedures should also be downgradable to in-line quality control processes, or at least cost-saving with respect to traditional high resolution characterisation techniques. Innovative HR resolution should be standardised. 3

4 RESIDUAL STRESS MEASUREMENT BY FIB A controlled removal of a small material volume is performed by FIB at low ion current ( pa); The attendant relaxation strain is then measured, step-by-step, by Digital Image Correlation (DIC) techniques; The residual stress (in plane distribution and depth profile) is then calculated from the measured strain by using either analytical or Finite Element modelling 4

5 IMPLEMENTATION OF FIB-DIC PROCEDURES Focused Ion Beam controlled material removal, by the ring-core milling geometry Digital Image Correlation for strain measurement Residual stress calculation Geometries Surface patterning FIB milling parameters SEM imaging parameters 2D strain mapping; Control of SEM noise Best patterning for DIC GUI FEM of influence functions for several geometries Models to correct for Elastic anisotropy FEM for through-thickness stress gradient analysis; 5 5

6 FIB MILLING GEOMETRIES Ring-core method Materials Letters 63 (2009) Slot-milling method Thin Solid Films 443 (1-2), pp Micro-cantilever method Acta Materialia 57 (6), 2010 Hole drilling method Nanotechnology 17, Number 20, 2006 H-bar Surface & Coatings Technology (2013) Marco Sebastiani FIB user EgFroUuGp 20143

7 KEY-WORD FOR INDUSTRY: AUTOMATION The whole test sequence is completed in ½ hour and multiple test positions can be set for overnight testing; DIC is performed live during test Only preliminary alignment and focus is required to the operator E-beam Pt deposition of surface features Automated patterning/imaging/milling 0 and 52 imaging after each milling step Automated Correction of ion/electron drift during the test; All test are initially set through a GUI Geometry Size SEM/FIB parameters 7

8 NEW PATTERNING STRATEGIES FOR IMPROVED DIC Reduced influence of the Pt-pad Full strain map over the X-Y plan Best strain measurement along the fast-scan direction More effective DIC (resolution of 1/1000 pixel) Freeware DIC runnign on Matlab (C. Eberl) 8

9 RESIDUAL STRESS CALCULATION PROCEDURES Bσ = ε σ 0 σ 1 σ 2 σ 3 = = or = σ 1 3 =0

10 Regularisation algorithms for stress depth profile calculation 9

11 THE ISSUE OF DEPTH MEASUREMENT We need to develop accurate calibration procedures for depth measurement. This is critical in view of an effective residual stress depth profile. 10

12 APPLICATION TO THIN FILMS FOR AVERAGE STRESS AND STRESS GRADIENT ANALYSIS IN THIN FILMS M. Sebastiani, C. Eberl, E. Bemporad, G. M. Pharr Materials Science and Engineering A, µm CAE-PVD CrN on steel AISI M2 11

13 CRITICAL ISSUES FOR STRESS DEPTH PROFILING DIC is performed LIVE during the test (e.g. during FIB milling after each SEM imaging step); Performing AT LEAST five test in a reasonable amount of time (1/2 day) DEPTH MEASUREMENT is a very critical issue in view of the full automation of the method! 12

14 DEPTH PROFILING OF RESIDUAL STRESS: CrN Strategy for stress gradient calculation: fitting with a polynomial expression of the strain data for h/d<0.3 Calculation of the residual stress DEPTH PROFILE by FEM analysis (Integral Method) Significant gradient of stress is observed from surface to interface on this CrN coating 13

15 LOCAL RESIDUAL STRESS FIELD IN THIN FILMS Surface and Coatings Technology, 2013 The results consistently show a significant modification of the LOCAL residual stress field induced by the droplet 14

16 LOCAL RESIDUAL STRESS FIELD IN THIN FILMS (ii) the induced coating renucleation process and modification of the growth mechanisms in correspondence of the defect Surface and Coatings Technology, 2013 How to explain the observed differences: (i) an additional thermal stress factor given by the presence of a metallic droplet 15

17 APPLICATION ON SUSPENDED MICRO-BRIDGES (MEMS RF SWITCHES) Sebastiani, M., Bemporad, Korsunsky, A.M. (2010) AIP Conference Proceedings, 1300, 16

18 APPLICATION ON SUSPENDED DOUBLE CLAMPED MICRO-BRIDGES del -400 m- ETD LIME Un1RomaTRE Sebastiani, M., Bemporad, Korsunsky, A.M. (2010) AlP Conference Proceedings, 1300,

19 APPLICATION ON SUSPENDED MICRO-BRIDGES RF SWITCHES , 450 lu Q.. 400!. 350 (/) (/) (II. ṭ.... (/) jij 200 :J "0 "Cii (II Stress variation over the suspended membrane 150 T 100 f y ' ' ' ' ' :' et:: 50 0 ta :sid membran Di s ' nce from one ' e of the ' e (urn) '!

20 APPLICATION ON AMORPHOUS PLASMA SPRAYED SINGLE-SPLATS (APPLICATION IN TBCS) Sebastiani, M., Bolelli, G., Lusvarghi, L., Bandyopadhyay, P.P., Bemporad, E. Surface and Coatings Technology, 206 (23), pp

21 APPLICATION TO AMORPHOUS MATERIALS Al 2 O 3 Surface and Coatings Technology, 206 (23), pp Single splats obtained by plasma spraying 20

22 Al splats: Relaxation strain from ring-core incremental milling 2.SE-03 Test-1 Test-2 2.0E-03 Test-3 cn : ;... Ul c: 0 n;...,>< q; l.se-03 l.oe _-.{ o Test-4 o Test-1on small splat without cracking o Test-2 on small splat without cracking Polynomial fitting S.OE-04 O.OE +OO :.::..._.u... -::::<: "' Millin g depth from surface ( nm ) o Small splats (no cracks) o Interpolated strain at 400 nm: o- 3 ± o- 3 o Average residual stress: ± 55.4 o Elastic modulus by nanoindentation

23 CONCLUSIONS FIB/DIC METHOD 1/2 A TOOL for micron-scale 3D residual stress analysis on a micron scale has developed and optimized. Good results The adopted geometry gives high depth resolution ( 100 nm) and lateral spatial resolution ( 1 µm) The developed milling strategies have improved the repeatability and robustness of DIC procedures LIVE strain depth profiling during FIBing and semiautomated procedure Calculation of the through thickness stress profile; 22

24 CONCLUSIONS FIB/DIC METHOD 2/2 Critical issues to be solved Full automation of the procedure. Does the FIB induces additional stresses?? Microstructure/anisotropy/plasticity effects?? Careful analysis of elastic properties for stress calculation. Cross validation with other high-resolution methods (synchrotron-xrd, EBSD, µ-raman). Ongoing work: establishment of a standardization activity through a European large project 23

25 ISTRESS EUROPEAN PROJECT EU project ISTRESS has started on Jan 1 st 2014 Pre-standardization of incremental FIB micro-milling for intrinsic stress evaluation at the sub-micron scale A new VAMAS TWA and project liaisons with CEN TCs are going to be established 24

26 ISTRESS PROJECT Some info at this webpage: MANY OPEN POSITIONS WITHIN THIS PROJECT: WE ARE LOOKING FOR FIB EXPERTS TO WORK IN THE CONSORTIUM! WE ARE LOOKING for associated partners to be involved in the ROUND-ROBIN activities. The STM group at Roma TRE: prof. Edoardo Bemporad prof. Fabio Carassiti Daniele De Felicis Marco Renzelli Federico Massimi Riccardo Moscatelli 25 Dragana Nikolic