SAM failure analysis using scanning acoustic … failure analysis using scanning acoustic microscopy...

| 1 | PVA TePla Analytical Systems GmbH

- C o m p a n y C o n f i d e n t i a l -

SAM failure analysis using scanning acoustic microscopy for diagnostics

of electronic devices and 3 D system integration technologies

Dr. Peter Czurratis1), Peter Hoffrogge 1), Dr. Sebastian Brand2), Frank Altmann2), Prof. Dr. Matthias Petzold2)

1) PVA Tepla Analytical Systems GmbH, Westhausen, Germany

2) Fraunhofer Institute for Mechanics of Materials IWM, Halle (Saale), Germany



Outline

• Motivation and Background

• Potential of Scanning Acoustic Microscopy

• Limitations & Challenges of SAM

• Technical Advances in SAM

• Analog pre-processing

• Optimized amplification

• New transducer design

• SAM in GHz-band (Resolution in the 1 µm scale)

• Results

• Summary



Motivation and Background

Background

- Ongoing requirements for non-destructive

methods in failure analysis

- failure localization, especially in z dimension

- investigation of failure - root cause

- high through put solutions

- NDT with highest possible resolution

Motivation

- current NDT methods are be limited

- improve imaging resolution

- penetration depth

Why Acoustic Microscopy ?

- operating non-destructively

- depth specific information

- high sensitive for voids and small

delaminated areas

cracks/inclusions in ceramic material



Potential of Scanning Acoustic Microscopy

- non-destructive investigations of

near all kinds of opaque materials

- non-destructive cross-sectioning

- high axial- and lateral resolution,

depending on frequency

- fast 3D-imaging

-estimation of E modulus, G modulus

and Poisson ratio

acoustic X-section of an IC

molded device

mold

die top

die bottom

lead frame

die attach



Current limitations & Challenges

Current limitations:

- frequency vs. penetration: acoustic attenuation within the materials

- penetration depth (lens aperture, focussing, frequency)

- resolution (wavelength depending on sound velocity)

- requires coupling fluid (impedance matching)

- throughput scanning speed

x1; yn x2; yn x3; yn xn; yn

x1; y1 x2; y1 x3; y1 xn; y1

2D-scan required

Attenuation vs. Frequency



How to overcome those challenges / limitations ?

ADC

PC

ultrasonic transducer

coupling fluid

silicon

BCB

interconnect wiring /

micro-bumps

∑T/R switch pre-amplifier pre-processing

line - signal

power-amplifier

increased transmit power,

customized design increased sensitivity of receiver chain / SNR, time corrected gain

performance of piezo-element (sputtered)

focus behavior of acoustic lenses,

application specific solutions

analog pre-processing

increased sensitivity of data

acquisition unit

software based post- processing

faster scanning/

multi-channel devices/

parallel scanning



Technical Advances in SAM

– analog pre-processing –

• pre-processing rf-data during scan

• analog -> extremely fast (real-time)

• HILBERT integration of signal

• low-noise components / increased SNR

Die bump interface with pre-processing Die bump IF without pre-processing




– optimized amplification –

• impacts sensitivity and signal to noise ratio

• restricts inspection of materials with high absorption / scattering

• decreases frequency (exponential relationship) -> directly linked to resolution

• increased transmit amplitude (amplification; transmit power)

• increased receiver-sensitivity

• decreased noise number of receiver chain

Acoustic attenuation :

Solutions :

Design of optimized transmitter/receiver electronics with 20 dB increase in gain




– reducing scanning times –

fully automated SAM inspection system

- Auto-Wafer- simultaneously scanning

4-channel-device

X axis:

Stroke: 450mm

Max. acceleration: 50 m/s2

Max. Speed: >2 m/s

High precision uni-directional

repeatability: +/-0,2µm

Y-axis:

Stroke: 450mm

Acceleration: 30 m/s2

open load port:

4,5,6,8 inch




– transducer design –

- optimized application specific transducer design and performance

- thin film technology for SAM: piezo elements sputtered on lens substrates

- application specific lens design

- numerical simulation of sound propagation / pressure distribution

- customized acoustic impedance net work of the lenses

piezo element

acoustic lens

sound diffraction pattern




– transducer design –

-thin film technology

-Piezo layers from 100 µm-1µm

-f=100 MHz-2000 MHz

-WD=10mm-40µm




– acoustic GHz-microscopy –

GHz-SAM control unit - 100 MHz – 2 GHz -Tone-Burst excitation

Extremely fast high-resolution scanner - 60 Hz line-repetition frequency

- 50 nm scan resolution - 2 mm scan field

- 10 mm defocus range

Opto- acoustic GHz microscope



Results



MEMS

bonded wafer

analysis



C-scan image of bonded wafers

B-Scan

C-Scan showing the location of the B-Scan (green line)

B-Scan

delamination

Top surface

Bond interface

Key issues: through put, defect detection and

resolution, automated defect analysis



10 µm

High resolution scans using fully automated systems: 30.000 pixel images for 12 inch wafers, 4 channels

Acqusition of >30.000 pixles/channel including DRT software: 5 µm pixel res for MEMS sensors and bonding inspection



Quantification of bonding interfaces

Example: Si-Si molecular bonding

10.3 10.4 10.5 10.6 10.7 10.8

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

entrance echo compressional wave

bonding interface transverse wave

bonding interface

t: 10,324 µs

A: 0.27

Δt: 0ns

t: 10,518 µm

A: 0.09

Δt: 194 ns

t: 10,666 µm

A: 0.125

Δt: 342 ns

t: 10,745 µm

A: 0.47

Δt: 421 ns

Sample Amplitude 1st echo (entrance) [V]

t-echo – 1 [µs]

Amplitude 2nd interface [V]

t-echo – 2 [µs]

Quotient A1/A2

6.1 0.303 10.292 0.09 10.537 3.36

6.2 0.337 10.33 0.085 10.504 3.96

6.3 0.299 10.323 0.080 10.519 3.73

6.4 0.297 10.328 0.080 10.52 3.7125

6.5 0.288 10.333 0.087 10.509 3.3

6.6 0.291 10.332 0.087 10.519 3.34



Surface Profilometry using

GHz-SAM



Results: Near surface investigations with µm Resolution

(lateral and axial)

10 µm

Acoustic micrograph:

Cr deposition on photo

mask for mask

repair applications

x [µm]

z [

µm

]

5 10 15 20 25 30 35

-90

-80

-70

-60

-50

-40

-30

V(x, z) graph

optical micrograph

6 µm

-5

-4

-3

-2

-1

0

1

2

3

topography parametric image

• V(z) scan @ 1GHz acoustic frequency

• highly focussed acoustic lens: f#=0.65

• lateral resolution: ~1 µm

• axial resolution: approx. 500 nm



Inspecting

µ-Bump Devices using

GHz-SAM



Results: µ – Bumps

Acoustic Inspection through BCB, inter connectors

BCB (5 µm)

Inter connect wiring

Silicon (800 µm)

µ-Bumps

acoustic inspection

interface to be Inspected: check for defects



Results: µ – Bumps

@ 1 GHz acoustic frequency resolution 1 µm

30 µm

x [µm]

z [µ

m]

20 40 60 80 100 120 140-90

-80

-70

-60

-50

-40

-30

20 µm

acoustic micrograph (surface in focus)

acoustic X-section recorded at increasing defocus positions

(transducer stepwise moved toward sample)

acoustic micrograph at

-10 µm defocus; imaging of

interfaces behind BCB key

arrows correspond to position

in X-section scan

X-section along red marker



20 µm

10 µm

µ - pillar

SAM image

SAM image

SEM image

- delaminations clearly visible

- 1 µm resolution

Results: µ – bumps delamination inspection

@ 1 GHz acoustic frequency resolution 1 µm



Delamination Analysis in Stacked Devices

using

post-Processing in SAM



device in flip-chip technology, BGA for electrical connections

connection defects : cracks disconnection

30 mm

20 m

m

80µm

75

0 µ

m1

00

µm

60

0 µ

m

lead frame: copper

chip: silicon

Investigations of flip chip devices



Micro-void detection

750 µm Si

Detection of cracks, inclusions, voids within the bumps and underfill



BAI and BAIn Analysis

digital filtering and pre-processing of raw echo signals

suppresses noise

amplifing of even small signal fragments

)(tS BAI

ThresholdifS BAI 0

ThresholdifSS BAI

n

BAI

2

1

)()(

t

t

BAI dttshilberttS

BAI – parametric imaging BAIn – parametric imaging

BAI (Back Amplitude Integral) signal processing :



Data Processing (BAI Analysis)

• RF acquisition: Z scans • sliding time gate analysis (80 ns) • backscattered amplitude integral (BAI)

BAI vs. gate position

void

1 21



BAI Slides



TO

F [

us]

position [um]3000 3500 4000 4500 5000

6.8

6.9

7

7.1

7.2

7.3

7.4

x 10-6

defect bump

Echo Separation Based Approach

additional back reflections



detected from acoustic data

verified by X-ray microscopy

verified by cross sectional SEM imaging

X-ray Microscopy: missed 6 BGA defects

Acoustic Microscopy: detected missed defects

Result Verification by SEM/XRAY/SAM



50 µm

SEM/XRAY/SEM cross section: micro voids, delaminations, inclusions



Velocity estimation of acoustic longitudinal and transverse wave modes using scanning acoustic microscopy and derivation of elastic property-

maps



Material properties

Z2

Z1

Z1

I0

IR1

IR2

IT1

IT2

d

dt

US-transducer derivatives :

density:

stiffness:

e-modulus:

shear modulus:

bulk modulus:

Poisson ratio:

l

l

Z

2

11 lc

2

44 sG c

44 11

44 11

1 2

2(1 )

c c

c c

11

(1 )(1 2 )

(1 )E c

11 44

4

3 B c c

preconditions:

• linear elastic

• homogeneous • isotropic • non- dispersive



Sample number Thickness

of SiC - layer

vlong

[m/s]

vshear

[m/s]

density

[g/cm^3]

E-modulus

micro hardness

[GPa]

E-modulus

micro

hardnes

[GPa]

E-modulus

(acoustic)

[GPa]

G-modulus

(acoustic)

[GPa]

1 890 µm 11700 7100 3.76 21.8 474 440 195

2 700 µm 10200 6800 4.19 17.73 455 422 193

3 1850 µm 10800 5200 4.19. 14.502 293 310 114

Esimation of mechanical parameters



Summary & Conclusions

• Need for NDT for ongoing improvement of SAM-equipment to increase performance

• Focus: optimization in electronics, transducers and software-tools

• Addressing signal power, penetration depth, attenuation, resolution and frequency

• Extending the fields of application for 3D-integrated devices:

• defect analysis in layered structures

• µ-bump inspection

• high resolution SAM inspection for thin layers

• fully automated control of IGBT modules, MEMS and bonded devices

• applicable in failure analysis and process control for 3D-manufacturing

• ongoing quantification of SAM measurements (E-modulus e.g.)

Technical advances

Results

Future Work

SAM failure analysis using scanning acoustic … failure analysis using scanning acoustic microscopy...

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