Scanning Acoustic GHz-Microscopy · © Fraunhofer-Institut für Mikrostruktur von Werkstoffen und Systemen IMWS Scanning Acoustic GHz-Microscopy Dr. Sebastian Brand, Ph.D. Fraunhofer

© 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


Agenda

Background and Motivation

Brief Introduction to Acoustic Microscopy

GHz-SAM

Summary and Conclusions


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


Introduction to

Scanning Acoustic Microscopy

– SAM –


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


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


Bonded Wafer Pair

Si

Si


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


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


Crack Inspection in Si/Al2O3 Wafer

Al2O3

Si

Compressional wave imaging

- defocussing required - excitation of transverse wave mode

Shear wave imaging


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


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


GHz – SAM

– Introduction & Examples –


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


Detection of sub-surface cracks


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


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


Inspection of Cu-Cu wire bonds


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)


Conventional SAM 300 MHz (FL 0.1“)

GHz – SAM 1 GHz (FL 80 µm)

Inspection of Cu-Cu Wire Bond interfaces


Stress induced Voiding


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

© Fraunhofer-Institut für Mikrostruktur von Werkstoffen und Systemen IMWS 23

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 –


Stress induced voiding in AlCu lines influence of TiN


Inspection of TSVs


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


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


Aiming at the inspection of TSVs by GHz – SAM

defocus sequence (line)


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


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


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


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

Documents

Scanning Acoustic GHz-Microscopy · © Fraunhofer-Institut für Mikrostruktur von Werkstoffen und Systemen IMWS Scanning Acoustic GHz-Microscopy Dr. Sebastian Brand, Ph.D. Fraunhofer