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Chip Scale Review July • August • 2017

[ChipScaleReview.com]

High-volume manufacturing (HVM) of chip-on-submount

(CoS): challenges and solutions

By Daniel F. Crowley, Peter Cronin

[MRSI Systems]

h e d ema nd f o r d a t a a nd

bandwidth continues to expand,

resulting in the requirement

for high-volume manufacturing (HVM) of

photonics devices at unprecedented levels.

This expansion, with double-digit growth rates,

is driven by data consumption from cloud

computing, web and mobile-based applications

and storage through hyperscale data centers

(e.g., Facebook, Google, Microsoft, Amazon).

The data bandwidth demand from individual

consumers and enterprises creates the need

to upgrade long haul networks, metropolitan

communication systems, and data centers.

One key photonic device component

requiring HVM is the chip-on-submount

(CoS). CoS devices present some unique

manufacturing challenges. Application-

specific die bonders have evolved over

decades to address the current manufacturing

demands of CoS. This article reviews the

required features of a successful HVM die

bonder of CoS devices.

The basics of a chip-on-submount (CoS)

The volume for photonic devices is

increasing for the reasons mentioned above.

Among the critical building blocks for

photonic devices, the CoS devices are the

core in terms of performance, reliability and

required volume.

In fiber optic transmission, the laser diode

(LD) CoS, also referred to as a chip-on-carrier

(CoC), is the starting point of light generation

and transmission, and the photodetector (PD)

CoS is the ending point of the light, which

is received and translated into an electronic

signal. It is quite typical for a 100Gbps fiber

transmission to have four sets of LD and PD

CoS at a rate of 25Gbps each. Some have 10

sets at a rate of 10Gbps each. The quantity of

LD CoS and PD CoS can therefore be four to

ten times the quantity of final optical modules.

A common configuration of a LD CoS,

includes a eutectically-bonded LD and a back

facet monitoring detector, both mounted on

the same submount. The quality of the joint

between the LD or PD chip to the submount

is one of the most critical factors for device

long-term reliability. Eutectic bonding is used

for a highly thermally efficient interconnect

with long-term reliability. The LD CoS may

include additional components, such as

thermistors, capacitors and driver chips. The

PD CoS typically includes a PD and may

also include additional components, such as a

transimpedence amplifier (TIA), and in some

cases, a thermistor for temperature control. In

subsequent process steps the completed CoS

may be mounted onto

a thermoelectric cooler

(TEC) packaged in a “gold

box” or TO-can package.

In order to achieve the

eutectic bonding of these

multiple parts within the

package, a temperature

hierarchy of eutectic

solder is frequently

required. When using a

TO-can package, it is common that the LD and

the monitoring detector are each eutectically

bonded on separate CoS and then mounted in

a vertical plane just offset from 90 degrees of

each other (

Figure 1

).

Die bonder requirements for HVM

of CoS

The majority of die bonders offered today

have evolved for generic semiconductor

packaging applications. While these

semiconductor die bonders may be well-

suited for the die bonding requirements

of semiconductor packages, they lack

the specific unique features required for

successful HVM of CoS.

Another approach to solving successful

HVM of CoS is to use dedicated highly

mechanical die bonders. While these dedicated

mechanical solutions can offer high speeds,

they often do so by sacrificing flexibility,

reliability and machine delivery times. Any

changes in the CoS design or type of the

devices being assembled result in expensive

and time consuming retooling costs.

The best solution is an application-specific

specialized die bonder for HVM of CoS that is

able to deliver high speeds while maintaining

flexibility. This is best achieved with a standard,

flexible, high-speed system that can be easily

reprogrammed for new device designs and

types. This approach results in shorter delivery

times for standardized machines. Short

machine delivery times are important to allow

customers to scale production rapidly. AHVM

CoS die bonder should include many parallel

processing features to maximize throughput.

The following is a summary of the unique

features of a CoS-specific HVMdie bonder.

Eutectic bonding optimized for CoS.

Because of high power requirements, small

component sizes, and the need for the

elimination of materials that outgas, many

components are attached with a eutectic die

bonding process. The CoS substrate sizes are

small, typically less than a few millimeters. A

small form factor eutectic stage is needed to

provide ultra-fast temperature ramping, and

ultra-fast cooling cycles. A closed-loop, low-

mass, high-power, pulsed heated eutectic stage

is required. Stable temperature control and

cover gas flow must be optimized throughout

the eutectic process. Additionally, the eutectic

station must be fully programmable, tailoring

the process for a specific bonding recipe.

Faster speeds, higher yields and repeatable

quality are the outcomes (

Figure 2

).

T

Figure 1:

LD CoS.

Figure 2:

Eutectic bonding stage.