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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. 1 06120 Halle/S. Germany ( + 49 (0) 345 5589-193 + 49 (0) 345 5589-101 * [email protected] www www.imws.fraunhofer.de
© Fraunhofer-Institut für Mikrostruktur von Werkstoffen und Systemen IMWS
Agenda
Background and Motivation
Brief Introduction to Acoustic Microscopy
GHz-SAM
Summary and Conclusions
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Background - need for non-destructive methods - failure localization - investigation of failure - root cause - limited # of samples (field returns) - comprehensive analyses required
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
Issues and challenges in 3D-Integration
µ bumps TSV‘s
inter-wiring interface delaminations
Motivation and Background
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Introduction to
Scanning Acoustic Microscopy
– SAM –
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Interface „A“
Additional Signals: reverberations, shear-long converted waves, etc. Reflected / Scattered Signals
contain information about interaction with sample structure
Interface „B“
Interface „C“
Interface „A“ Interface „B“
Interface „C“
Introduction to SAM
power
am
p
x
y
z
water container
- Ultrasonic Transducer
- X/Y/Z – stage
- Insonation at each lateral position
- Reception and analysis of echos
- Echos contain information of
interaction wave - material
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1
1 2
2(90 )
ZT
Z Z
2 1
1 2
(90 )Z Z
RZ Z
Z1
Z2
r
e
tl
ts
transmitted component
reflected component
converted component (excitation of other wave modes depending on the angle of incidence and the materials elastic properties)
incident component
Z vt
ijcv
Z = Acoustic Impedance (mode specific)
1 1
2 2
sin( )
sin( )
v
v
v1
v2
Contrast Mechanism in Acoustics
0 10 20 30 40 50 600.7
0.75
0.8
0.85
0.9
0.95
1amplitude of reflectance function
angle of incidence [deg]
refl
ecta
nc
e a
mp
litu
de
[a
.u.]
2
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Bonded Wafer Pair
Si
Si
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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
Focuss ing in SAM is rather difficult
Sound Intensity Fields
water S i behind a layer of water
Acoustic Lens Acoustic Lens
S i
water
water
Focus of Sound Beam
Significant LensParameters: - Radius of Curvature - Opening Angle - Acoustic Frequency
Focal Plane
ϕ
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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
Wave Modes in GHz-SAM
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Crack Inspection in Si/Al2O3 Wafer
Al2O3
Si
Compressional wave imaging
- defocussing required - excitation of transverse wave mode
Shear wave imaging
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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
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Limitations & Challenges
Limitations:
- acoustic attenuation
- penetration depth (lens aperture, focussing)
- resolution (wavelength depending on sound velocity)
- requires scanning
- requires coupling fluid (impedance matching)
x1; yn x2; yn x3; yn xn; yn
x1; y1 x2; y1 x3; y1 xn; y1
Attenuation vs . Frequency
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GHz – SAM
– Introduction & Examples –
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Conventional SAM vs. GHz – SAM
Parameter Conv. SAM GHz – SAM
max. Resolution 15 µm > 0.85 µm
Frequency Range 5 – 300 MHz 0.4 – 2 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
conventional SAM
GHz – SAM
acoustic attenuation
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Detection of sub-surface cracks
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40 µm40 µm
optical micrograph GHz-SAM micrograph (in focus) GHz-SAM micrograph (de-focussed)
- artificial defects induced by nano-indentation - GHz-SAM inspection in V(x,y,z) mode
GHz – SAM for sub-surface µ-crack detection
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SE- micrograph
40 µ
m
defocussed GHz-SAM - micrograph
clear indication for crack
PFIB - cut 50 µm
defocussed GHz-SAM - micrograph
surface GHz-SAM - micrograph
GHz – SAM for sub-surface µ-crack detection
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Inspection of Cu-Cu wire bonds
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Inspection of Cu-Cu Wire Bond interfaces
Ultrasonic signal
SAM inspection from chip back side
Si chip
Leadframe
BEOL stack
Die attach
pad metal
Chip front side
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)
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Conventional SAM 300 MHz (FL 0.1“)
GHz – SAM 1 GHz (FL 80 µm)
Inspection of Cu-Cu Wire Bond interfaces
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Stress induced Voiding
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22 schematic of sample cross-section
3 different sets of samples investigated
S1, S2 and REF
S1 and S 2 stressed by 1000 TCT cycles between -65°C and 175°C
(according to “JESD22-A104E: Temperature Cycling”; AEC-Q100 Revis ion G)
Increased ohmic resistance observed in sample type S1
Reason: Induction of voids -> reduction of el. cross-section -> impact on reliability
Investigation of the effect of a 40 nm TiN interlayer
Sample type: S1, REF Sample type: S2
Stress induced voiding in AlCu lines
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FIB – trenching of sample S1 (+ ESD artefacts)
SEM imaging of metal lines
Corresponding features marked by equal colors
in GHz-SAM and SEM images
GHz - SAM SEM
Stress induced voiding in AlCu lines – Verification –
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Stress induced voiding in AlCu lines influence of TiN
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Inspection of TSVs
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Time-Resolved vs. Time integrated Acquisition in GHz-SAM
- spatial oversampling
and averaging
- fast
- high SNR
- V(z) applicable
- can be s low
- SNR can be low
seq. averaging
- V(z) applicable
- spectral content
preserved
- s ignal analys is ,
feature extraction
and parametric
imaging
Time-resolved data acquis ition currently under development
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Time-Resolved vs. Time integrated Acquisition in GHz-SAM
Box-Car Integration vs. Time-Resolved
20 µm
20 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
empty TSVs: : 5 µm depth: 50 µm
spatial averaging reduces imaging detail
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Aiming at the inspection of TSVs by GHz – SAM
defocus sequence (line)
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Aiming at the inspection of TSVs by GHz – SAM
60 µm
-12µm
60 µm
-14µm 955MHz
910MHz
- 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
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Sound intensity in S i substrate with void
void
couplant
Highly limited penetration depth @ 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 signals @ 1GHz
Identification of individual modes
TSV likely acts as a wave guide
TSV with void
Si S i
H2O
Lens
GHz-SAM – Transient Simulation of Wave Propagation –
TSV with edge delamination
Si
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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
© Fraunhofer-Institut für Mikrostruktur von Werkstoffen und Systemen IMWS
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. 1 06120 Halle/S. Germany ( + 49 (0) 345 5589-193 + 49 (0) 345 5589-101 * [email protected] www www.imws.fraunhofer.de
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