Silicon Photonics: A Platform for Integration, Wafer Level Assembly and Packaging

M. AsghariKotura Inc

April 2007

2

Contents:

► Who is Kotura

► Choice of waveguide technology

► Challenges and merits of Si photonics packaging

Integration: High functionality and lower component count

Fibre interface

Stress related issues

Thermal management

Hybrid assembly

► Examples of current activities

3

Who is Kotura?

► Founded in 2003: Merger of two companies

► A Leading Silicon Photonics CompanyShipping in volume to Tier-1 customers

Products deployed in live networks

Over 50 patents issued / pending and license to all Bookham Si based IP.

► Strategy based on partnershipBuild competence on chip design and fabrication

Be a high value, high functionality chip supplier

Form partnerships for product development, fast market penetration and achieving maximum application coverage

► In house Si Fab, prototype packaging and low volume A&T

0.25um DUV Scanner, World Class Dry Etch Capability, ...

► Sub Con manufacturing partner in China

► Happy to work as partner / foundry on a joint development or sub-contract basis

4

Si Technology: Basics

Material Property Si

Refractive Index 3.47 Well controlled

Material Type Crystal

dn / dT 1.8x10-4K-1

Thermal Conductivity, K 130-156 W/mK

Tuning 85 pm / oC

Temp Stability + / - 0.1 oC

Key Strengths- Manufacturability- Scalability- Volume cost curve- Process control- Potential for integration- Solid state reliability- Electronic integration- Full set of functionality

Passive devicesCurrent injection devices

Detectors, VOAs etc

Hybridisation capability

Key Challenges- Fibre interface- Temperature control- Stress issues on packaging

Si

Si substrate

Oxide

Waveguide width

Silicon thickness

Ridge height

5

Choice of Waveguide Technology

Photonics sweet spot

0.01

0.1

1

10

100

0.1 1 10

Deep etch technology

Bend Radius (mm)

Propagation Loss (dB/cm)

Experimental data

Waveguide Dimension (um)

0.01

0.1

1

10

100

Ben

d R

adiu

s (m

m)

Prop

agat

ion

Loss

(dB

/cm

)

Kotura

FiberIII/VElectronics

6

Choice of Waveguide Technology

► Propagation loss independent of Wavelength and Polarization

► Very low group velocity and polarization mode dispersion

► Very low waveguide crossing loss and X-talk► Optimum core size for most optical

functionalities is at 3 to 4um.

0.5 1 1.5 2 2.5

2

3

4

WG Thickness (um)

Effe

ct a

nd g

roup

inde

x

Group index

Effective index

TETM

TE TM

-0.3

-0.2

-0.1

0

1520 1530 1540 1550 1560 1570

Wavelength (nm)

Propagation Loss (dB)3um Core Waveguide

Experimental

TE(dB/cm)TM(dB/cm)

-120

-100

-80

-60

-40

-20

0

0 1 2 3 4 5 6 7 8 9Waveguide Width, W ( um)

Cro

ssta

lk (d

B)

101.8

1.5

1.2

0.9

0.6

0.3

0

Loss

(dB

)

XTalk_TE(0.5um)XTalk_TE(1um)XTalk_TE(3um)Loss_TE(0.5um)Loss_TM(0.5um)Loss_TE(1um)Loss_TM(1um)Loss_TE(3um)Loss_TM(3um)

H

Theoretical

H

Loss

X-Talk

0.5

1

3

0.5

13

7

Passive Waveguide Technology

Key ComponentsBends and MirrorsCouplers and splittersMode transformersWavelength Mux / Demux / selective elementsPolarisation splitters, rotators and discriminatorsOptical / modal Filters

-40

-35

-30

-25

-20

-15

-10

-5

0

191.8 192.3 192.8 193.3 193.8 194.3 194.8 195.3 195.8

Frequency (THz)

Tran

smis

sion

(dB

)

R

Part 1Part 2

Part 3

<0.1dB Loss for U-bend with 250um Radius

22--

24

24--

26

26--

28

28--

30

30--

32

32--

34

34--

36

36--

38

38--

40

40--

42

42--

44

44--

46

46--

48

48--

50

50--

52

52--

54

54--

56

56--

58

Intra-channel Crosstalk (dB)

Num

ber

of Com

bin

atio

ns.

0

100

200

300

400

500

8

Current Injection Devices: VOA example

n-type

p-type

p+ n+

Silicon

BOX

0

5

10

15

20

25

30

35

40

45

50

0 5 10 15 20 25 30 35 40 45 50

Current (mA)

Atte

nuat

ion

(dB

)

01234567

0 2 4 6 8 10

Attenuation (dB)

Phas

e Sh

ift

(uni

ts o

f Pi)

00.10.20.30.40.50.60.70.80.9

1

0.0001 0.001 0.01 0.1 1 10

Time (ms)

Nor

mal

ised

pha

se c

hang

e

SOI Thermal Thin BOX 400mW

SOI - Carrier injection 10 mW

Silica 400mW

SOI Thermal Thick BOX

40mW

-22-20-18-16-14-12-10-8-6-4-20

0 5 10 15 20 25 30 35 40Applied Current / mA

Extin

ctio

n / d

B

Bar TE Cross TEBar TM Cross TM

MZ switch

9

III/V Device Hybridization

Si MetalizationVertical plinth

wirebond

Solder

Si Waveguide

Horizontal taper in Si can improve lateral coupling loss due to lateral placement in-accuracies

Shelf on which the laser is mounted

Metal track

Accurate vertical alignment is achieved by removing low tolerance layers from the laser and accurate etching in Si of mounting shelfAlignment marks aim to achieve maximum lateral placement accuracySi plated with solder and includes some simple electronics componentsOn placement of laser local heating is applied to reflow the solder

Solder

MirrorWaveguide

Photo-diode Hybridization

Laser / SOA Hybridization

Commodity actives for low

cost

High Resisitivity Si for RF Lines

10

Fiber Coupling

• A three dimensional taper (mode expander) reduces coupling loss to <0.5dB.• The back reflection from such a facet is <-50dB• PDL is <0.05dB• Wavelength and Temperature insensitive• Technology offers low cost passive V-groove based fibre attach

Kotura’s Patented Epi Taper technology enables: - Fiber mode matching with no compromise in device performance- Core size reduction evolution

3D Taper

-70

-60

-50

-40

-3020 30 40 50 60 70

Vertical Fibre position (um)

Back

Ref

lect

ion

-65dB

1um to 4um core

12um core

11

Stress Management

Ex field Ey field

Strain not included

Ex field Ey field

Typically 0.1% TM

Stress induced x & y index

change

Stress and roughness can cause mode conversion

Strain included

~0.0% TM

Challenges:

► Si is prone to stress induced index change

► This can cause polarization conversion, polarization mode dispersion and PDL

► A combination of stress and roughness can also cause mode conversion

12

Stress Management

After PDF Compensation

-12-11.5

-11-10.5

-10-9.5

-9-8.5

-8-7.5

-7-6.5

-6

191.8 191.85 191.9 191.95 192 192.05 192.1 192.15 192.2 192.25 192.3 192.35 192.4 192.45 192.5 192.55 192.6 192.65 192.7 192.75 192.8 192.85 19

-10.5-10

-9.5-9

-8.5-8

-7.5-7

-6.5-6

-5.5-5

-4.5

192.8 192.85 192.9 192.95 193 193.05 193.1 193.15 193.2 193.25 193.3 193.35 193.4 193.45 193.5 193.55 193.6 193.65 193.7 193.75 193.8 193.85 19

Before PDF Compensation

Benefits► Stress can be used for performance

correction eg on AWG array for PDF.► Stress can be utilized for sensor

applications

0

0.5

1

1.5

2

2.5

3

3.5

0 10 20 30 40

Channel

PDF (GHz)

13

Thermal Considerations

► High Si dn/dT requires a high level of thermal stability

► High thermal conductivity of Si enables excellent temperature uniformity and effective heat removal

► Developments in Si packaging have enabled better than 0.1oC thermal management even with high power and time varying thermal dissipation at chip level.

► Integrated temperature sensors can be fabricated into the fabric of the Si chip close to sensitive areas

-600

-400

-200

0

200

400

600

-10 0 10 20 30

40 50 60 70

Ambient Temperature / DegC

Grid

Offs

et /

pm

Improvements made in thermal management of Si PLCs

0

2000

2500

010 20 30 40 50 60 70 80

Temperature (deg C)

Resistance (Ohms) nin: 0.5% °C-1, pip: 0.7%°C-1

DT = 0.37 oC

10 channel VOA

3 channels on

9 channels on

Temp differential on chip, DT = 0.38 oC

14

Thermal Considerations ….

-4

-3

-2

-1

0

1

2

3

4

0 10 20 30 40 50 60 70

Temperature (C)

Wavelength Shift (nm)

DFB (0.1)DFB(0.08)AWG (Si)AWG (SiO2)

Thermal isolation trench

► Thermal isolation features such as deep etches and bridge structures can be fabricated to enable thermal isolation

► Si and InP based devices have very similar wavelength drift with temperature

15

Si Integrates Waveguides and Micro-Bench Capabilities

Laser: Passive Auto-aligned

Passive Fiber Attach

VOAs

Surface Mount PD

Front Facet Monitor

Grating or WDM Coupler

- Wafer scale testing

- Flip chip technology for fiber attachFuture Development

WDM Coupler

WDM Coupler

Monitor VOA Laser

VOA

PIN

APD

Active area

p+

n+BOX

Developed in collaboration with Enablence

16

100Ge CWDM Solution

Tx► 10 CWDM DFB Lasers hybridized into the Si

chip either individually or as a bar► Integrated AWG or Echelle grating, to Mux

channels together► Lasers directly modulated► Front facet detectors integrated to provide

power monitoring► VOAs can be integrated to provide power

balancing without the need to modify laser drive conditions

Rx► Individual PINs or a bar, surface mounted

onto wet etched 54 degree vertical coupling mirrors.

► Demux is integrated into the same chip ► Fast VOAs can be integrated to improve

dynamic range or provide protection

Lasers Detectors

DFB λ registration: +/- 2nm

12nm6.5nm

17

Kotura’s CWDM 100Ge Solution

Au:Sn solder

Active area

WDM DFB Array(Cyoptics)

Mux / DeMux

PD / Surface mount technology

Monitor / VOAs

p+ n+

BOX

- High level of integration both monolithic (waveguide) and hybrid (micro-bench)- Developed in partnership with Cyoptcis into a very small form factor package

VOAs

15mm

7mm

18

Fitting 100G into ~10G Footprint !

100Ge Module

100Ge TxRx

10 x 10G ROSAs

Micro-bench assembly

Si PLC Tx Chip 100G TOSA

MuxDe Mux+ +

10 x 10G TOSAs

Or

Si PLC Rx Chip

Conventional technology can not support the footprint

Si Solution can support 10G FXP footprint for 100G and offers huge cost saving potential

100G ROSA

&

19

Key Challenges & Merits

► Key ChallengesLow propagation and fiber coupling lossFacet preparationOptical, Electrical & Thermal X-talkLow stress chip attachThermal management

► Key meritsLow component count: IntegrationLow cost fiber attach: V-grooveStress engineering for compensationThermal tuningExcellent thermal conductivity and temperature uniformitySmall foot printHighly scaleable and cost effective manufacturing: CMOS fabLow cost, automated A&T: Wafer scale hybridization and test, Peak and place assembly, Lead frame packaging…

Lead frame

Simple assembly

Ceramic Carrier

Fiber Block

Si AWG

20

Reliability and Stability

Damp Heat (85C - 85% RH)in-situ monitoring 40 VOA channels

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

1000 1100 1200 1300 1400 1500

Time (hours)

Del

ta IL

(dB

)

500 hr for qualification and 2,000 hrs for information

Si is a proven Technology platform:- Well known material system- Solid state switching …

Mea

n lif

e (3

,000

hr t

est)

( ) )()( 11 nnTtTuk

E RuRtaeAF −+−=Ayring acceleration model

Mean life over 35 years

Predicted mean life based on 85/85 life testsa=0.0005, n=2, 0.024 kg water per kg dry air

0

20

40

60

80

100

120

140

160

180

200

0.2 0.4 0.6 0.8 1.0 1.2

Activation Energy [eV]

Acc

eler

atin

fact

or [-

]

0

10

20

30

40

50

60

70

80

90

100

AF (2,500Hr)

ML [yr]

Real proof is:- Real & qualified products - Tier 1 customers- In live networks carrying traffic

Reliability is always a key barrier to deployment of a new technology- Higher integration increases the possibility and consequences of failure- Takes time and a lot of hard data