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39

Chip Scale Review November • December • 2016

[ChipScaleReview.com]

Dicing silicon carbide power devices using thermal

laser separation

By Hans-Ulrich Zuehlke, Mandy Gebhardt

[3D-Micromac AG]

his paper will give an overview

of the potential of a laser

dicing approach, called TLS-

Dicing™, for SiC-based semiconductor

products. In the described case study,

a typical power device wafer with full

backside metallization, polyimide and

metal structures in the dicing streets was

separated by using this new technology.

A high yield count in combination with

very high edge quality was demonstrated.

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

significantly reduced.

Challenges with dicing SiC power

devices

Silicon carbide (SiC) is a crystalline

compound of silicon and carbon. It

possesses certain qualities such as high

charge carrier mobility, high strength,

hardness, and high thermal conductivity.

Due to these characteristics, SiC is

considered a replacement material

for silicon (Si)-based semiconductors

in the electronics industry in certain

applications, including power devices,

light emitting diodes (LEDs), and sensors

for harsh environments. Cost, quality

and throughput are all major factors

in achieving successful manufacturing

of SiC-based devices. Wafer dicing is

evolving as a critical value-add process

step that not only ensures, but further

enhances, SiC device yields.

T h e t r a d i t i o n a l t e c h n i q u e f o r

separating SiC devices from wafer form

is mechanical blade sawing. This method

involves a very fast rotating foil with

abrasive particles to remove the wafer

material. Because of the hardness of SiC

(Mohs scale 9.2), blade sawing suffers

from low feed rate and high wear of the

diamond-coated dicing blade, resulting

in the risk of uncontrolled tool breakage

during the dicing process. In addition,

blade sawing can result in chipping

and delamination at the edge of the die

(

Figure 1

). An advanced version of

this approach is ultrasonic vibration-

supported mechanical sawing, which

provides slightly higher dicing speed

(10-20mm/s), but in principle has the

same limitations. With the upcoming

transition of SiC wafer sizes from

4-inch to 6-inch diameters, mechanical

blade dicing will reach its limit because

the cumulated street length more than

doubles and is beyond the ability of

one saw blade to completely cut. In this

situation, the blade would either have to

be changed while the wafer is in work-

position or, in the worst case scenario,

the blade will break during the middle of

the dicing process and damage the wafer.

Laser ablation is an alternative approach

to wafer dicing, but when applied to

SiC wafers it can result in micro cracks,

significant heat affected zones, and metal

debris in the streets, all of which can

impact die yields.

Thermal laser separation overview

The rma l l a s e r s epa r a t i on (TLS-

Dicing) is a fast, clean and cost-effective

alternative to separating SiC-based

semiconductor products. It has many

advantages compared to competing

technologies such as blade sawing and

laser ablation:

• Hi gh s e p a r a t i on s p e e d ( up t o

300mm/s for SiC) resulting in a

throughput of 10 wafers per hour

(assuming a 4-inch wafer with 2mm

die size);

• Very smooth edges (avoids increased

leakage current of vertical diodes by

leaving the p/n-junction undamaged);

• Nearly no chipping and micro cracks

for less breakage;

• Thin backside metal on the chip is

separated without damage;

• No tool wear;

• Low cost-of-ownership due to no tool

wear and nearly no consumables; and

• Zero kerf cleaving for reduced

street width.

The new thermal laser separation process

uses thermally-induced mechanical stress to

cleave brittle semiconductor materials such

as SiC, Si, germanium (Ge) and gallium

arsenide (GaAs). A laser heats up the solid,

brittle material and generates a zone of

compressive stress, surrounded by a zone of

tangential tensile stress pattern. Next, a jet of

extremely small amounts of deionized (DI)

water spray is applied that creates a second

cooled zone with a minimum distance to

the first one—inducing a tangential tensile

stress pattern. The resulting tensile stress in

the overlaying region of both stress patterns

has a local maximum that is sharply focused

and has a clear orientation (perpendicular to

the street), and is therefore able to open and

guide the crack tip through the material.

The new process is always a one-

pass process that separates the whole

thickness of the wafer at once. The

starting point is given by a shallow

scribe that is either local or continuous

at the wafer’s surface. The local scribe

is preferred to ensure the highest bending

strength and least particle generation.

On the other hand, the continuous scribe

offers best results for products with

metal in the street and improves the

straightness of the cleaving process.

T

Figure 1:

SiC die edge after mechanical dicing shows

chipping and delamination defects [1].