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Chip Scale Review September • October • 2017

[ChipScaleReview.com]

Nondestructive 3D X-ray imaging for advanced

packaging failure analysis

By Cheryl Hartfield

[Zeiss Semiconductor Manufacturing Technology]

, and Daniel Nuez

[Xilinx, Inc.]

a c k a g i ng of i n t e g r a t e d

c i r c u i t s i s g r o w i n g

more and more complex

– a n d h o u s i n g mu l t i p l e d i e i n a

single package is just one challenge

ch ipma ke r s f a ce. Ty pica l ly, t he s e

die are connected in complex ways,

a n d c h i p m a k e r s m u s t c o n t e n d

w i t h s h r i n k i ng f e a t u r e s i z e s a nd

i n t e r c o n n e c t s , e s c a l a t i ng d e v i c e

den sit y a nd pa ckage si ze, t h i n ne r

l a y e r s , a n d a w i d e n i n g v a r i e t y

of materials.

As a result, failure analysis (FA)

on advanced packages is becoming

increasingly difficult. The goal of FA is

to isolate where the failure is located,

and then figure out what it is and why it

happened – its root cause. Visualization

of defects aids determination of the

root cause. Packages are essentially

opaque boxes cont ai ni ng elect r ical

con nect ions. Of t en , t o v isual i ze a

defect in the electrical path, physical

failure analysis (PFA) is applied.

Maintaining integrity of the defect

site is critical. If a sample is cut or

reduced i n si ze, f u r t he r elect r ical

a na lysis may not be pos sible, a nd

t he st r uct u re may be d isr upt ed by

introducing artifacts or changing the

stress prof ile from that of an intact

sample. Conventional nondestructive

methods have become less effective at

visualizing defects in many of today’s

packages, creating a significant need

for new nondest r uctive approaches

such as 3D X-ray microscopy (XRM).

Benefits of X-ray microscopy

In the typical board- and package-

level FA lab workf low, failures are

e v a l u a t e d nond e s t r u c t i ve l y p r i o r

to dest r uct ive analysis (

Figure 1

).

The mos t common nonde s t r uc t ive

PFA t e ch n i que s fo r i s ol a t i ng a nd

v i s u a l i z i n g d e f e c t s a r e o p t i c a l

inspection, 2D X-ray, and scanning

acoustic microscopy (SAM). Due to

increased package complexity, these

imag i ng t e ch n ique s a r e be comi ng

less effective.

X R M , a r e l a t i v e l y n e w FA

t e c h n i q u e , u n i q u e l y p r o v i d e s a

h i g h - r e s o l u t i o n , n o n d e s t r u c t i v e

met hod t o f i nd a nd image defe c t s

i n 3D. It t he reby prov ide s c r it ical

k n ow l e d g e t o g u i d e n e x t s t e p s .

Applicat ion of XRM t ypically f it s

between fault isolation and root cause

determination (

Figure 1

).

Once the fault location is isolated,

traditionally, a next step is a visit to

the “coroner’s office” – that is, PFA

techniques that dest roy the sample

are used to investigate the root cause

of the failure. The techniques cited

on the far right side in

Figure 1

all

involve physically cut ting, d r illing

or otherwise altering the sample in

some way. If the fault is not properly

located , t here is no second chance

t o f i nd it u n le s s a not he r pa ck age

is sacrificed.

Providing 3D intelligence ahead of

destr uctive analysis is a key benef it

of XRM. It enables higher success

rates in cross-sectioning and finding

root causes. Visualization of defects

b y 3D XRM c a n e v e n e l i m i n a t e

t he ne e d t o p e r fo r m PFA , s av i ng

time and resources. The case st udy

i ncluded i n t h is a r t icle i l lu st r at e s

the effectiveness of 3D XRM in the

FA workf low.

Visualizing defects nondestructively

with virtual cross sections

T h e p owe r o f 3D t omo g r a p h y

come s f r om it s abi l it y t o p r ov ide

virtual cross sections, revealing the

det ails i nside st r uct u res.

Figure 2

provides a simplified overview of the

XRM tomog r aphy process.

Figure

2a

shows t hat d at a is acqu i red by

collecting 2D projection images from

a rotating sample positioned between

an X-ray source and a detector (the

yellow dot in

Figure 2a

). The XRM

detector is composed of scintillator-

coupled optical microscope objectives

c omb i n e d w i t h a c h a r ge - c ou p l e d

dev ice (CCD) came r a . The X-r ays

pass through the sample and hit the

scintillator mounted on the objective

l e n s . T h e s c i n t i l l a t o r c o n v e r t s

t he pat t e r n r e s u lt i ng f r om X- r ays

transmitted through the sample into

t he opt ical image capt u red on t he

right (

Figure 2b

). The sample is then

rotated slightly, the image captured

again, and this process is repeated

through up to 360 degrees of rotation.

The resulting group of projections –

typically, between 1,000 and 2,000 –

are then processed by algorithms to

mat hemat ically reconst r uct t he 3D

volume (

Figure 2c

).

The t ime requ i red for t he ent i re

p r o c e s s i s v a r i a b l e – t y p i c a l l y

rangi ng between 30 mi nutes and 8

hou r s – depend i ng on t he numbe r

of project ions and how much t ime

P

Figure 1:

Acceptance of 3D X-ray microscopy is growing for failure analysis.