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17

Chip Scale Review September • October • 2018

[ChipScaleReview.com]

High-resolution 3D X-ray microscopy for

semiconductor advanced packaging measurements

By Cheryl Hartfield, Allen Gu, Raleigh Estrada

[ZEISS Semiconductor Manufacturing Technology, PCS Strategic Business Unit]

he demand for i nc rea sed

miniaturization and higher

I /O i n mobi le a nd h ig h -

per formance devices has d r iven an

explosion of innovations that enable

high-density multi-chip architectures,

including but not limited to 2.5/3D

memor y and logic packages, wafer-

level packages (WLP), and system-

i n - p a c k a g e ( S i P ) . I n c r e a s i n g l y,

these designs are driving packaging

interconnects into the third dimension.

Me a s u r eme n t s a t c r it ic a l p r o c e s s

s t e p s a r e a k e y e n a b l e r f o r t h e

introduction and production of new

and advanced technologies that have

narrow process margins or are difficult

t o cont rol. Because t he st r uct u re s

i n t od ay’s a dva nced pa ck age s a r e

of ten i naccessible or too small for

me a s u r eme nt s by common ly u s e d

nondestructive methods (for example,

2D X-ray), new approaches are required

to accomplish these measurements with

a practical throughput to enable the

fastest time-to-market for new products.

Submicron 3D X-ray microscopy

(XRM) offers a new approach to enable

rich volumetric and linear measurements

i n a s t r e aml i ne d way t h a t c a n b e

automated and without the need for

time-consuming, manual cross sections.

Figure 1

shows the power of analyzing

a bu l k volume i n 3D (

F i gure 1b

)

versus analyzing a plane of structures

(

Fi gure 1a

) [1]. The re a re g reat e r

numbers of structures accessible in the

3D volume, enabling better measurement

st at ist ics and oppor t un it y for new

t ypes of measurements that are not

possible from 2D cross section images.

Additionally, 3D XRM can provide

be t t e r me a su r ement r e pe at abi l it y,

especially as the continued shrink of

str uctures accelerates the diff iculty

to perform a cross section accurately

t o a d e s i r e d me a s u r eme n t p l a n e

w h i l e s i mu l t a n e o u s l y a v o i d i n g

polishing artifacts.

XRM measurement of advanced

package structures

A key advantage of 3D XRM aiding

both failure analysis and measurement

applications is the ability to image any

plane from any angle within the virtual

3D dat aset. Defects and st r uct u ral

va r iat ions can be obser ved f rom a

desired view and exactly in the plane

of interest. The observation is typically

accomplished by viewing successive

virtual cross sections through manual

interaction with the data set. Because it

is a virtual data set, any cross-sectional

slice may be viewed at any point in the

3D volume from any direction. This

offers tremendous benefit over physical

cross sectioning, where only one cross-

sectional plane is available to view [2].

For the application of measurements,

after acquiring the XRM tomographic

data, a semi-automated measurement

work f low is used to ext ract usef ul

structural information from the sample.

Involving multiple steps, the workflow

is of ten scr ipted in a measu rement

template to speed up measurement and

eliminate human errors. The workflow is

described below.

Fi r s t , s u r f a c e s of s t r uc t u r e s of

interest are precisely extracted by a

local adaptive algorithm. Second, the

structures to be measured in the 3D data

are registered in a coordinate scene by

object alignments. Third, key geometric

elements and features are created on the

registered volume. Finally, the relevant

me a s u r eme n t r e s u l t s a r e d i r e c t l y

extracted and reported. The result is not

limited to linear or 2D metrics. Various

volumetric parameters and 3D features

can be generated through this workflow.

Because the measu rement template

can be repeated ly applied to ot her

data, automation is possible to enhance

measurement efficiency.

The 3D reconstructions from XRM

scans provide rich data for statistical

analyses, which are vital to ramping

t h e ma n u f a c t u r i n g y i e l d o f n ew

complex a dva nc ed pa ck age s . Fa s t

nondestr uctive measurements allow

process integration engineers to look at

data from statistically relevant sampling

percentages from a manufacturing line.

XRM 3D reconst r uctions and thei r

data can also be stored for archival

purposes. If required, 3D volumetric

measurements can be converted by the

software to 2D linear measurements

t o a l low fo r compa r i s on s t o l a t e r

destructive measurements.

Case study: CMOS image sensor

package for smartphone cameras

CMOS image sensor (CIS) devices are

small and efficient parts of the cameras

in mobile handsets, with most advanced

T

Figure 1:

a) SEM image of a physical cross section;

b) 3D X-ray microscopy image of the same structures.

3D images enable direct volumetric measurements in

addition to linear measurements.