Scanning Acoustic GHz-Microscopy

Transkript

1 Scanning Acoustic GHz-Microscopy Dr. Sebastian Brand, Ph.D. Fraunhofer Center for Applied Microstructure Diagnostics CAM Fraunhofer Institute for Microstructure of Materials and Systems Department of Microelectronics and Microsystems Characterization of Semiconductor Technologies Team Leader Non-Destructive Techniques Walter-Huelse-Str Halle/S. Germany ( + 49 (0) (0) * Sebastian.Brand@imws.fraunhofer.de www

2 Agenda Background and Motivation Brief Introduction to Acoustic Microscopy GHz-SAM Summary and Conclusions

3 Motivation and Background Background - need for non-destructive methods - failure localization - investigation of failure - root cause - limited # of samples (field returns) - comprehensive analyses required interface delaminations inter-wiring Motivation - current NDT methods may be limited (not applicable in 3D integration) - imaging resolution - penetration depth - contrast mechanism Why Acoustic Microscopy? - operating non-destructively - depth specific information - sufficient(?) penetration depth - sensitive to mechanical properties - excitation of different wave modes µ bumps TSV s Issues and challenges in 3D-Integration

4 Introduction to Scanning Acoustic Microscopy SAM

5 Introduction to SAM z x - Ultrasonic Transducer - X/Y/Z stage y power amp - Insonation at each lateral position - Reception and analysis of echos water container - Echos contain information of interaction wave - material Interface A Interface B Interface C Interface A Interface B Interface C Reflected / Scattered Signals contain information about interaction with sample structure Additional Signals: reverberations, shear-long converted waves, etc.

6 reflectance amplitude [a.u.] Contrast Mechanism in Acoustics incident component reflected component transmitted component R(90 ) Z Z Z Z v 1 converted component (excitation of other wave modes depending on the angle of incidence and the materials elastic properties) T(90 ) 2Z12 Z Z 1 2 Z1 Z2 e r tl Z = Acoustic Impedance (mode specific) Z v t v c ij v 2 ts amplitude of reflectance function sin( ) sin( ) v v angle of incidence [deg]

7 Bonded Wafer Pair Si Si

8 Focussing of Acoustic Waves - Concave shaped lens (Saphire, Quartz) - Large differences in wave propagation velocities - Large refractive indices -> low aberrations - High reflectivity -> small signals - Angular incidence -> lateral wave modes Focussing in SAM is rather difficult water Acoustic Lens Sound Intensity Fields Si behind a layer of water Acoustic Lens Significant LensParameters: - Radius of Curvature - Opening Angle - Acoustic Frequency water Si Focal Plane ϕ water Focus of Sound Beam

9 Wave Modes in GHz-SAM longitudinal mode transverse mode direction of density/wave component change direction of wave propagation direction of density/wave component change direction of wave propagation Materials that support both modes also support Rayleigh waves Rayleigh Wave: Surface Acoustic Wave with longitudinal and transverse components Important in Acoustic Microscopy Animation courtesy of Dr. Dan Russell, Kettering University

10 Crack Inspection in Si/Al2O3 Wafer Al2O3 Compressional wave imaging Si - defocussing required - excitation of transverse wave mode Shear wave imaging

11 GHz-SAM V(x, y, z) Scan Sequence - scan planes (x,y) at multiple z - positions - lens with large aperture (100 aperture angle) - defocus leads to - angular insonation (exc. of SAW s) - focus below surface - high sensitivity to surface and subsurface - low penetration depth: approx. 1.5 Rayleigh - high acoustic attenuation in GHz-band - analog signal pre-processing -> increase SNR

12 Limitations & Challenges Limitations: - acoustic attenuation - penetration depth (lens aperture, focussing) - resolution (wavelength depending on sound velocity) - requires scanning - requires coupling fluid (impedance matching) Attenuation vs. Frequency x 1 ; y n x 2 ; y n x 3 ; y n x n ; y n x 1 ; y 1 x 2 ; y 1 x 3 ; y 1 x n ; y 1

13 GHz SAM Introduction & Examples

14 Conventional SAM vs. GHz SAM Parameter Conv. SAM GHz SAM max. Resolution 15 µm > 0.85 µm Frequency Range MHz GHz Penetration Depth large approx. 1.5 λ (3 20 µm) Line Frequency < 1 Hz < 50 Hz Focal Length 1.3 mm 25 mm 40 µm 250 µm Scan Range 400 x 400 mm 2 x 2 mm Transducers all single element transducers highly focussed acoustic lenses GHz SAM acoustic attenuation conventional SAM

15 Detection of sub-surface cracks

16 GHz SAM for sub-surface µ-crack detection - artificial defects induced by nano-indentation - GHz-SAM inspection in V(x,y,z) mode optical micrograph GHz-SAM micrograph (in focus) GHz-SAM micrograph (de-focussed) 40 µm 40 µm

17 GHz SAM for sub-surface µ-crack detection SE- micrograph surface GHz-SAM - micrograph 50 µm defocussed GHz-SAM - micrograph PFIB - cut 40 µm clear indication for crack defocussed GHz-SAM - micrograph

18 Inspection of Cu-Cu wire bonds

19 Inspection of Cu-Cu Wire Bond interfaces pad metal Chip front side BEOL stack Si chip Die attach Leadframe Acoustic Insonation 1. Wet-chemical removal of leadframe and die attach 2. Grinding of Si chip in package to target thickness (e.g. < 200 µm) 3. Total removal of Si by selective plasma etching (for very short focal lengths used for GHz-SAM) Ultrasonic signal SAM inspection from chip back side

20 Inspection of Cu-Cu Wire Bond interfaces Conventional SAM 300 MHz (FL 0.1 ) GHz SAM 1 GHz (FL 80 µm)

21 Stress induced Voiding

22 Stress induced voiding in AlCu lines 3 different sets of samples investigated S 1, S 2 and REF S 1 and S 2 stressed by 1000 TCT cycles between -65 C and 175 C (according to JESD22-A104E: Temperature Cycling ; AEC-Q100 Revision G) Increased ohmic resistance observed in sample type S 1 Reason: Induction of voids -> reduction of el. cross-section -> impact on reliability Investigation of the effect of a 40 nm TiN interlayer Sample type: S 1, REF Sample type: S 2 schematic of sample cross-section 22

23 Stress induced voiding in AlCu lines Verification FIB trenching of sample S 1 (+ ESD artefacts) SEM imaging of metal lines Corresponding features marked by equal colors in GHz-SAM and SEM images GHz - SAM SEM 23

24 Stress induced voiding in AlCu lines influence of TiN

25 Inspection of TSVs

26 Time-Resolved vs. Time integrated Acquisition in GHz-SAM - spatial oversampling and averaging - fast - high SNR - V(z) applicable - can be slow - SNR can be low seq. averaging - V(z) applicable - spectral content preserved - signal analysis, feature extraction and parametric imaging Time-resolved data acquisition currently under development

27 Time-Resolved vs. Time integrated Acquisition in GHz-SAM Box-Car Integration vs. Time-Resolved empty TSVs: : 5 µm depth: 50 µm conventional Box-Car integrated data acquisition newly developed time-resolved data acquisition TSVs appear all dark in time integrated acquisition differeing contrast observed with time-resolved data acquisition spatial averaging reduces imaging detail

28 Aiming at the inspection of TSVs by GHz SAM defocus sequence (line)

29 Aiming at the inspection of TSVs by GHz SAM -12µm -14µm 955MHz 910MHz m µ 06 m µ 06 - FIB-SEM verification of GHz-SAM results - TSVs showing indication in GHz-SAM correspond with defect at the top of the Cu - TSV with orange marker shows large void approx. 4 µm beneath surface

30 GHz-SAM Transient Simulation of Wave Propagation Highly limited penetration 1 GHz f# 0.77 Contrasts observed in TSVs -> investigation of the phenomenon Understanding of wave propagation in such complex situation (multi-material system, large gradients, large angles, curved wave fronts) Modelling and Simulation by Mode conversion, reflection, diffraction, interference Computation of received acoustic 1GHz Identification of individual modes TSV likely acts as a wave guide Lens H 2 O couplant Si Si void Si TSV with edge delamination TSV with void Sound intensity in Si substrate with void

31 Summary and Conclusions - Acoustic Microscopy is a powerful technique for applications in microelectronics failure analysis and metrology - Non-destructive micro-imaging through opaque materials - GHz-SAM is highly sensitive to surface and sub-surface regions - Provides high contrast for voids and inclusions (material properties) - Inspection of TSVs is rather challenging and requires further research - Continuing developments for extending GHz-SAM applicability - Arbitrary wave generation - Time-domain acquisition spectral analysis of acoustic signals and feature extraction parametric imaging

32 Scanning Acoustic GHz-Microscopy Dr. Sebastian Brand, Ph.D. Fraunhofer Center for Applied Microstructure Diagnostics CAM Fraunhofer Institute for Microstructure of Materials and Systems Department of Microelectronics and Microsystems Characterization of Semiconductor Technologies Team Leader Non-Destructive Techniques Walter-Huelse-Str Halle/S. Germany ( + 49 (0) (0) * Sebastian.Brand@imws.fraunhofer.de www