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Chip Scale Review November • December • 2016

[ChipScaleReview.com]

Because thermal laser separation is

a cleaving process, the die edges are

smooth and free of remaining stress

or micro cracks and chipping zone

(

Figure 2

). Any reduction in bending

strength as a result of the dicing process

is significantly lower compared to

ablative laser technologies. In addition,

the backside metal is separated with no

delamination or heat affects.

Experimental set-up

An experiment was conducted to

evaluate the impact of thermal laser

s e p a r a t i on on S i C d e v i c e y i e l d s .

T h e w a f e r s e p a r a t i o n w a s d o n e

with a microDICE™ system using

thermal laser separation technology.

The processed 4-inch wafer had a

thickness of 110µm and a thin silver-

type backside metal stack. The streets

were covered with several metal test

structures. The active zone of the dies

was protected by a polyimide coating.

On the frontside of the wafer, the outer

part of the edge exclusion zone was

covered by a ring of metallization

and polyimide coating. On account

of customer requirements, larger dies

were simulated by building 3 by 3

die-clusters of smaller dies. For this

reason, only every third street was

diced (

Figure 3

).

As described earlier, each cleave

n e e d s a d e f i n e d s t a r t i ng po i n t i n

the form of a local initial scribe. To

remove the reflecting metal structures

in the street, a shallow continuous soft

scribe along the entire street length was

applied. The scribing laser is a 532nm

short-pulsed laser with a Gaussian

b e am p r o f i l e . Sc r i b i ng s p e e d s o f

50mm/s up to 200mm/s were achieved.

The lower the scribing speed, the

smaller the heat affected zone of the

polyimide cover layer and the particle

po l l u t i on . Fu r t h e r i nv e s t i g a t i on s

have shown that comparable scribing

r e s u l t s c an be a ch i eved wi t h l e s s

expensive near infrared (NIR) laser

sources. A z-axis autofocus was used

to obtain a smooth scribing line. The

cleaving itself was realized by a 200W

continuous wave (cw) laser with NIR

wa v e l e n g t h . Th e c on s ump t i on o f

cooling liquid (DI-water) was below

10ml/min.

Experimental results

The new dicing method was able

to separate all chips without chipping

and micro cracks (

Figure 2

). There

is nearly no thermal impact of laser

machining to the polyimide cover

layer. No washing or cleaning and

no protective coating was applied –

a significant advantage in terms of

operation costs. The separation of the

backside metal was very smooth and

without any delamination (

Figure

4

). This is important for minimizing

problems during die mounting on the

heat sink. The applied continuous

scribe was used to open the metal

structures without any impact on the

bulk material after cleaving (

Figure 5

).

Yield impact

The processed wafer was analyzed

for defects and yield using a high-

resolution optical inspection tool. This

inspection method was adopted from a

best practice of an industrial SiC device

manufacturer. Yield consideration only

includes dies inside the edge exclusion

zone. The average yield value for this

application is more than 98.9%. The

observed defects and deviations have

been categorized (

Table 1

). Nearly all

of the defects are located in the edge

exclusion region.

A noted characteristic pattern of the

cleaving process was a short, local

cleave dislocation, perpendicular to

the edge of the wafer. In all cases,

t h i s d e v i a t i on wa s l imi t e d t o t h e

edge exclusion region (

Figure 6

) and

can be ignored for yield calculation.

For a few chips, the cleave was not

perfectly straight, but the deviation

never reached the active area of any

Figure 2:

SiC die edge after the TLS-Dicing process

shows smooth edges and no micro cracks or chipping.

Figure 3:

The whole wafer was diced; and target die size was simulated by 3 by 3 clusters of smaller dies.

Figure 4:

Smooth separation of backside metal, with

the typical 4-degree off orientation visible.

Figure 5:

The photo shows the effect of metal in

the dicing street. Separation is successful with minor

impact on straightness as noted by the Wallner lines.