Acoustic GHz-Microscopy: Potential, Challenges and
Applications
A Joint Development of PVA TePLa Analytical Systems GmbH
and Fraunhofer IWM-Halle
Dr. S ebas tian Brand (Ph.D.)
Fraunhofer CAM
Fraunhofer Institute for Mechanics of Materials
Department of Microelectronics and Microsystems
Characterization of Microsystems
Acous tic Micros copy
Walter-Huelse-Str. 1
06120 Halle/S.
Germany
(

*
www
+ 49 (0) 345 5589-193
+ 49 (0) 345 5589-101
Sebastian.Brand@iwmh.fraunhofer.de
www.iwmh.fraunhofer.de
© Fraunhofer-Institut für Werkstoffmechanik IWM
Agenda
The Acoustic GHz-Microscope
-
Features and Requirements
-
Microscope Hardware and Technical Background
Potential and Challenges
-
Tray & Stitch Mode
-
V(z) and Time-integrated Mode
-
Time-resolved Acquisition
-
Wave Modes and their Use
-
Challenges
Selected Applications
Summary and Outlook
© Fraunhofer-Institut für Werkstoffmechanik IWM
The GHz-SAM
and
the Acoustic High-Resolution Lens
A Joint Development of PVA TePLa Analytical Systems GmbH
and Fraunhofer IWM-Halle
© Fraunhofer-Institut für Werkstoffmechanik IWM
Parameter
Value
Resolution
> 0.65 µm
Frequency Range
Penetration Depth
Line Frequency
0.4 – 2 GHz
approx. 1.5 λ
< 50 Hz
Focussing
f#: 0.57 – 1
Refractive Index
approx. 7.5
acous tic GHz-lens
© Fraunhofer-Institut für Werkstoffmechanik IWM
The Acoustic GHz-Lens
Purpose :
- Active element (piezo) excites elastic wave
- Plane wave spherically focussed (cavity)
- Wavelength and geometry of cavity define
shape of sound field
- Achievable lateral resolution dependent
on sound field
SF in 2D
lateral cut through SF in focus
© Fraunhofer-Institut für Werkstoffmechanik IWM
axial cut through SF
GHz-SAM – Technical Background
Communication
TRIGGER
Gate
l
na
l
na
si g
eo
si g
rf-
vi d
z - STAGE
y - STAGE
x - STAGE
PULSE
Fast High-Precision
Scanner
Receiver Unit
Motion Controller
Pulser Unit
RECEIVE SIGNAL
Control Center
sample
© Fraunhofer-Institut für Werkstoffmechanik IWM
GHz-SAM – Technical Background
Device developed by
and
CAM
© Fraunhofer-Institut für Werkstoffmechanik IWM
The GHz - SAM
- Scanner mounted on optical microscope
- Scan line repetition frequency (LRF) up to 50 Hz
- Extremely fast frame acquisition (15s – 30s)
- Scan range can be defined freely 50 µm – 2 mm
- Scan resolution can be defined freely down to 100 nm
- Combined with precise X-/Y-stage for sample positioning
- Automated scan sequencies
© Fraunhofer-Institut für Werkstoffmechanik IWM
Additional Features / Options of the GHz-SAM
Alternativ e ex citation S ignal by Arbitrary Wav eform Generation
- Spike excitation
- Chirp signals for high axial resolution
- Customized broad-band pluses
Acquis ition of the unproces s ed RF-data
- Signal analysis using SAMNALYSIS
- Extraction / Identification of Wavemodes
- Estimation of wave velocities
© Fraunhofer-Institut für Werkstoffmechanik IWM
Potential
and
Challenges
© Fraunhofer-Institut für Werkstoffmechanik IWM
Tray - Scan
- Max. Lateral Range of Scanner : 2 mm @ 30 Hz LRF
- Sample re-positioning by precise X/Y- stage
- Fully automated scan sequence
- Scan field extension by TRAY – Scan
- Resulting data stitched to form global image
© Fraunhofer-Institut für Werkstoffmechanik IWM
V(z) - Scan
- 2D-Scan repeated at multiple z-Positions
- Max. z-Range of Scanner : 2 mm
- Advantage of defocus sequence
- Acquisition of optimum image
- Excitation of SAW
- Fully automated scan sequence
© Fraunhofer-Institut für Werkstoffmechanik IWM
Wave Modes in GHz-SAM
longitudinal mode
direction of density/wave
component change
direction of wave propagation
transverse mode
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
Animation courtesy of Dr. Dan Russell, Kettering University
© Fraunhofer-Institut für Werkstoffmechanik IWM
GHz-SAM – V(x, y, z) Scan Sequence
© Fraunhofer-Institut für Werkstoffmechanik IWM
Use of V(z) Approach
V(z) curve at a broad band excitation
10
V(z) curve at a single frequency
20
amplitude [dB]
10
frequency [MHz]
30
Δz
0
-10
40
50
60
-20
70
-30
80
90
-40
-7
-6.5
-6
-5.5
rel. defocus position [µm]
z 
sin(R ) 
0
2(1  cos(R ))
v0
vR
V0 = sound velocity of coupling
vR = Rayleigh wave velocity
θR = Rayleigh angle
© Fraunhofer-Institut für Werkstoffmechanik IWM
-6.8
-6.6
-6.4
-6.2
-6
rel. defocus position [µm]
-5.8
-5.6
Challenges in Acoustic GHz-Microscopy
- acoustic attenuation
- penetration depth (lens aperture, focussing)
- resolution (wavelength depending on sound velocity)
- requires scanning
- requires coupling fluid (impedance matching)
acous tic attenuation
x 1 ; yn
x1 ; y1
x2 ; y1
x2 ; yn
x3; y1
x3; yn
xn; yn
xn; y1
2D-s can required
© Fraunhofer-Institut für Werkstoffmechanik IWM
Selected Applications
© Fraunhofer-Institut für Werkstoffmechanik IWM
GHz-SAM Inspection of a microbolometer device
40 µm
leg
Potential problem area (void?)
contact window
leg
contact window
© Fraunhofer-Institut für Werkstoffmechanik IWM
GHz-SAM Inspection of a microbolometer device
microbolometer decive
cut - A
-
transfer bonded thermistor onto
read-out-integrated-circuit
cut - A
cut - B
-
Imaging through layer of 2 µm
SiO2/Si/TiAl/SiO2
-
voids 2.5 µm beneath surface
cut - B
Bonding interface
pixel leg
reflector
thermistor
© Fraunhofer-Institut für Werkstoffmechanik IWM
GHz-SAM Inspection of Induced µ-crack
- 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
© Fraunhofer-Institut für Werkstoffmechanik IWM
40 µm
Defocus Sequence in GHz-SAM – V(x,y,z)
minor subsurface
crack
large,
obvious crack
features way
beneath surface
(10 µm)
© Fraunhofer-Institut für Werkstoffmechanik IWM
GHz-SAM Inspection of sub-surface µ-crack
surface GHz-SAM
- micrograph
SE- micrograph
50 µm
PFIB - cut
40 µm
defocussed GHz-SAM
- micrograph
clear indication for crack
© Fraunhofer-Institut für Werkstoffmechanik IWM
defocussed GHz-SAM - micrograph
Void Inspection on TSV‘s
GHz-SAM near surface image
V(x, z) of TSV – „A“
V(x, z) of TSV – „B“
sample courtesy by J. Wolf, Fraunhofer ASSID
© Fraunhofer-Institut für Werkstoffmechanik IWM
-
contrast in defocussed GHz-SAM image
correlates With voids found in TSV‘s
-
dimensions of TSV larger than theoretical
penetration depth
-
likely interface wave excited between
TSV and Si
-
further research required for clarification
Transient Simulation of Wave Propagation in GHz-SAM
-
1 GHz -> enormous computational effort
Mode conversion, reflection, diffraction, interference
Computation of received acoustic signal @ 1GHz
Identification of individual modes
TS V w ith edge delam ination
Lens
Lens
H2 O
Si
couplant
Air
Lens
H2 O
void
Si
TS V w ith v oid
Si
© Fraunhofer-Institut für Werkstoffmechanik IWM
S ound intens ity in S i s ubs trate w ith v oid
GHz-SAM visualization of sub-surface features
µ-Bumps
BCB (5 µm)
Silicon (800 µm)
interconnect wiring
20 µm
© Fraunhofer-Institut für Werkstoffmechanik IWM
GHz-SAM visualization of sub-surface features
30 µm
-30
-40
20 µm
z [µm]
-50
-60
-70
-80
-90
© Fraunhofer-Institut für Werkstoffmechanik IWM
20
40
60
80
x [µm]
100
120
140
GHz-SAM in Life Science
© Fraunhofer-Institut für Werkstoffmechanik IWM
GHz-SAM in Life Science Cell Thickness Estimation
Cell Thicknes s
© Fraunhofer-Institut für Werkstoffmechanik IWM
S ound Velocity
Summary and Outlook
Summary

Developed an acoustic GHz microscope

fast an precise scanner;

stable RF-chain

combined with optical microscope

successfully employed in first applications
Outlook

Quantitative parameter estimation (Z, E, G, ν)

Signal Analysis for parametric imaging

Application specific and Coded Excitation

Design of application specific GHz-lenses

Optimized Matching Layers improve wave transmission

Numerical simulation for investigating the propagation behaviour
in specific structures and geometries
© Fraunhofer-Institut für Werkstoffmechanik IWM
Acoustic GHz-Microscopy: Potential, Challenges and
Applications
A Joint Development of PVA TePLa Analytical Systems GmbH
and Fraunhofer IWM-Halle
Dr. S ebas tian Brand (Ph.D.)
Fraunhofer CAM
Fraunhofer Institute for Mechanics of Materials
Department of Microelectronics and Microsystems
Characterization of Microsystems
Acous tic Micros copy
Walter-Huelse-Str. 1
06120 Halle/S.
Germany
(

*
www
+ 49 (0) 345 5589-193
+ 49 (0) 345 5589-101
Sebastian.Brand@iwmh.fraunhofer.de
www.iwmh.fraunhofer.de
© Fraunhofer-Institut für Werkstoffmechanik IWM