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16

Chip Scale Review November • December • 2017

[ChipScaleReview.com]

Evaluation of advanced probe cards for large-array

fine-pitch micro-bumps

By Erik Jan Marinissen, Ferenc Fodor, Bart De Wachter

[imec]

and Jörg Kiesewetter, Ken Smith, and Eric Hill

[FormFactor]

ertically-stacked multi-

die assemblies are a cost-

attractive alternative to

keep the momentum of Moore’s Law

going now that technology-node

scaling becomes increasingly difficult

and expensive. Such assemblies come in

many flavors: 2.5D- and 3D-stacked ICs,

fan-out wafer-level packages (FOWLP),

etc. The stacked components are often

large dies in leading-edge technology

node s . Cons equen t l y, t he i r y i e l ds

require testing before stacking (so-

called “pre-bond test”), to prevent low

compound stack yields and associated

high costs. These products have in

common that their inter-die connections

are implemented with large arrays of

fine-pitch micro-bumps. For the non-

bottom dies in a die stack, these micro-

bumps are the only available wafer-

probe interface for pre-bond testing.

Until recently, probing large-array

fine-pitch micro-bumps to get wafer-

level pre-bond test access into the

dies was considered “impossible;”

conventional cantilever-type probe

cards cannot cover the large arrays,

and vertical probe cards do not offer

t h e r e qu i r e d f i n e p i t c h . Th i s h a s

changed with the arrival of advanced

MEMS-type probe cards that offer

l a rge f i ne - p i t ch p r obe a r r ays t ha t

match the micro-bumps. In this article,

we describe the process and results of

the evaluation of such advanced probe

cards at imec. For this purpose, we

have manufactured dedicated micro-

bump test wafers and developed and

installed inside our ø300mm Fab-

2 clean room a new test system with

dedicated hardware and software.

Probe targets

Today’s most challenging micro-bump

probe targets are specified by JEDEC

memory interface standards: High-

Bandwidth Memory (HBM2) has the

largest array (~4,900 micro-bumps) and

Wide-I/O Mobile DRAM (WIO2) has the

finest pitch (40µm). In our experiments,

we have used WIO2, as it has the most

aggressive pitch. A WIO2 interface

(

Figure 1

) consists of four banks of 73×6

= 438 micro-bumps each (hence, a total

of 1,752 micro-bumps) with bank gaps of

two rows and 24 columns [1].

Imec’s process-of-reference (PoR)

m i c r o - b ump s a t 4 0 μm p i t c h a r e

representative for the industrial state-

of-the-art. The landing bumps are made

of copper, have a diameter of 25μm,

and a height of 5μm. The top bumps are

made of copper, nickel, and tin, have

a diameter of 15μm, and a height of

5+1+3.5 = 9.5μm.

For characterization purposes, imec

designed dedicated test wafers containing

only micro-bumps which are all shorted

by an underlying blanket copper layer.

We named the die design “BMB:” blanket

micro-bump. Each of these ø300mm

wafers contains over 9.4M probe-target

micro-bumps, in addition to dummy,

identification, and alignment micro-

bumps. The 93 dies per wafer contain,

among others, 27 WIO2 arrays. We have

ma nu f a c t u r e d t h e s e

BMB w a f e r s w i t h

micro-bumps varying in

diameter and metallurgy,

i n c l u d i n g o u r P o R

micro-bumps.

A u t o m a t i c t e s t

system

The automatic test

system we have built

u p f o r m i c r o - b ump

probing in the clean-

room of imec’s Fab-

2 i n L e u v e n ( s e e

Figure 2

) consists of the following

components: 1) test instrumentation

consisting of a digital multi-meter

and a wide switch matrix, contained

in 2) a hard-docking test head with

manipulator, which connects through an

interface with spring-loaded contacts to

3) an advanced MEMS-type probe card,

which is placed in 4) a fully-automatic

probe station with wafer loader; the

system is completed by 5) in-house

developed software for automatic test

generation and result data visualization

and ana l y s i s . The va r i ou s s y s t em

components are described in more

detail below.

The probe station is FormFactor’s

fully-automatic Cascade CM300 in

dual configuration, i.e., two probers

sharing a central material handling unit

(MHU), a.k.a. “auto-loader.” The two

probers are largely identical. Both can

automatically load wafers from the

shared MHU, but also have a front-

side manual load port that accepts

ø200mm and ø300mm wafers as well as

large SEMI G74-0669-compliant tape

frames for wafers up to ø300mm [2,3].

V

Figure 1:

JEDEC’s WIO2 micro-bump array [1].

Figure 2:

Test system for evaluation of micro-bump probe cards installed in

Fab-2 at imec.