ISP part 2

1. AIM
Academy
Part 1:
Context
 Electronic-Photonic Integration
 Confinement
Part 2:
Passive
Devices
 Waveguides
Off-Chip Couplers
 Wavelength Division Multiplexing
Part 3:
Active
Devices
 Detectors
 Modulators
 Light Sources and Lasers
 Integrating Photonics

2. AIM
Academy CMOS Fabrication
CMOS Logic Platform
− Technology node
− Silicon substrate (bulk, SOI)
− Oxide gate, SiO2
− Salicide contacts, CoSi2
− Planarization
− Interconnect levels
− Vdd – Voltage drain-drain
3 Areas of Interest
- Substrate
- FEOL to PMD
- BEOL – Metallization
Integration Priorities
- FET Performance
- Thermal cycling-proper sequencing
- Cross Contamination
- Process complexity
IMD: Inter-Metal Dielectric
PMD: Pre-Metal Dielectric
FEOL: Front-End-Of-Line
BEOL: Back-End-Of-Line
*Salicide spike anneal 1050°C
 PMD – SiO2, Planarized
1.0mm
1.1mm
CMOS FET & Interconnect for 0.18 mm node
 Silicon Substrate p-type
 Gate, S/D junctions
 Metal – AlCu, Local
interconnect levels 1-4
 IMD – SiO2, Planarized
 Contacts – W studs
 Vias – W studs
900
<550
<450
750*
 Salicide, Ti, Co

3. AIM
Academy
Passive Photonics:
Waveguides
 Si-compatible
 Ultra-low loss in the IR
 Single mode design
 Losses: Measurement, Theory
 SOI, a-Si
 SiON, Si3N4
Substrate/Underlayer
SiO2 tunderclad
SiO2
200 nm
500 nm
~3.5 mm
~3 mm
Si

4. AIM
Academy Waveguide Materials
Core Si Si-rich
Si3N4
Si3N4 SiON
n 3.5 2.2 2.0 2.0-
1.445
n 2.055 0.755 0.555 0.555-0
 Dielectric waveguides
Core:
 Si: SOI, amorphous (a-Si)
 Si-rich Si3N4 / Si3N4: PECVD, LPCVD
 SiON: PECVD
Cladding:
 Undercladding: wet thermal SiO2
 Overcladding: PECVD SiO2
 Geometry
Strip
 h=0.2-1.0 mm (SOI, thin film)
 w=0.5-2.0 mm (UV-sub-UV lithography)
Ridge/rib
 h ~ 1 mm, trench=0.5-0.8 mm
 w= 1.0-5.0 mm
waveguide core refractive indices
SiO2 cladding is assumed in all cases.
Index difference: n2-0.01

5. AIM
Academy Waveguide Loss
a = S a = a side roughness + a top roughness + a bulk + a substrate
Si Substrate
 Substrate Leakage – f (n, h, w, tunderclad)
 Absorption – f (a bulk, n, h,w)
 Roughness Scattering – f (n, h, w, , Lc)
 Surface loss
 Sidewall loss
 Design > isolation design rules
 Material & process method
 CMP
 Etch & Post etch treatments
Sidewall scattering often dominates: TM mode lowest loss
abulk=acore+(1-)acladding
: power confinement factor
: roughness Amplitude
Lc: roughness correlation length
D. K. Sparacin, “Process and Design Techniques for Low Loss Integrated Silicon Photonics,” MIT Ph.D. Thesis (2006).

6. AIM
Academy
Metrology
Fabry-Pérot Method
 work with fewer samples: spectral scan
 FSR: peak spacing; loss/length: peak/valley ratio
 a priori knowledge of insertion loss: reflectivity calc deviates for n
 need accurate measure of waveguide length
d
Input
Output
~1.7 mm mode
field diameter
S.J. Spector et al., Proc. (2004).
D. K. Sparacin, “Process and Design Techniques for Low Loss Integrated Silicon Photonics,” MIT Ph.D. Thesis (2006).
S. Spector et al., Optical Amplifiers and Their Applications/Integrated Photonics Research, Technical Digest, IThE5 (OSA, 2004).
E(z  d,t) = Aei0 z
teid
t  (Aei0 z
teid
)reid
reid
t  (Aei0 z
teid
)reid
reid
 reid
reid
t  ... eit
=
Aei(0 zt)
eid
(1 r)2
1 x
, x  r2
ei2d
T =
E(z  d,t)
E(z,t)
2
=
eid
(1 r)2
1 r2
ei2d
2
a == imagimagreal i ;

7. AIM
Academy Si SOI Waveguides
 h=220 nm
 w=445 nm
Y.A. Vlasov and S.J. McNab, “Losses in single-mode silicon-on-insulator strip waveguides and
Bends,” Optics Express, v.12(8), pp. 1622-1631(2004).
aTE
TE 0.3 dB/cm
H. Rong, A. Liu, R. Jones, O. Cohen, D. Hak, R. Nicolaescu, A. Fang and M. Paniccia,
“An all-silicon Raman laser,” Nature, v.433(7023),pp.292-294 (2005)
1.5 mm SOI
 w=1.5 mm
 depth=0.7 mm
Ridge SOI waveguide
— w=1 mm
— w=5 mm
 aTE=3.6 ± 0.1 dB/cm
Si SOI: “photonic wire”

8. AIM
Academy
aTE=0.32  0.05 dB/cm
S. Spector, M. W. Geis, D. Lennon, R. C. Williamson, and T. M. Lyszczarz, " Hybrid multi-mode/single-mode waveguides for low loss," in
Optical Amplifiers and Their Applications/Integrated Photonics Research, Technical Digest (CD) (Optical Society of America, 2004), paper IThE5.
Mode-Engineered SOI Waveguides
 Couple light to first-order mode
 Multimode straight waveguide segments
 power remains confined to first-order mode
 minimal (TE) overlap with sidewalls
 Turns: adiabatic taper to single-mode waveguide

9. AIM
Academy
Conduction
Band
Valence
Band
Conduction
Band
Valence
Band
Conduction
Band
Valence
Band
Conduction
Band
Valence
Band
Conduction
Band
Valence
Band
Amorphous Silicon (a-Si) Waveguides
Dangling Bond States Absorb Light
 Extremely attractive as upper-level waveguide material
 Previous Reports of ~ 300 dB/cm Waveguide Loss
 H-passivation of a-Si reduces dangling bond states
 H-concentration in PECVD deposited a-Si materials is
high, should have low loss
D. K. Sparacin, “Process and Design Techniques for Low Loss Integrated Silicon Photonics,” MIT Ph.D. Thesis (2006).

10. AIM
Academy a-Si Waveguide, SiN clad
R. Sun, K. McComber, J. Cheng, D. K. Sparacin, M. Beals, J. Michel and L. C. Kimerling, “Transparent amorphous silicon channel waveguides with
silicon nitride intercladding layer,” Appl. Phys. Lett. v.94(14), p.141108 (2009)
TE-polarization, ring resonator measurements
Sample Resonance
wavelength (nm)
Extinction ratio
(dB)
-3 dB bandwidth
(pm)
Q factor Loss
(dB/cm)
1 1558.146 12.3 69.4 22452 12.0 ± 1.8
2 1559.587 5.4 25.8 60449 6.5 ± 0.9
3 1560.319 6.9 11.0 141847 2.7 ± 0.4

11. AIM
Academy Slot Waveguides
 Low index slot regions: potential host matrix for optically active dopants (Er,
nanocrystals)
 Access to effective index/group index values intermediate to Si3N4  Si waveguides
R. Sun, P. Dong, N. Feng, C.Y. Hong, J. Michel, M. Lipson, L.C. Kimerling, “Horizontal single and multiple slot waveguides: optical transmission at =1550 nm,” Opt. Exp. v.15(26), p.17967 (2007).
Expt Theory
74.6 pm/K 76.8 pm/K
65.4 pm/K 64.6 pm/K
Single Slot
Triple Slot
Si: 102.7 pm/K
dTd /

12. AIM
Academy
Passive Photonics:
Off-Chip Coupler
 Size mismatch of optical modes
 Index mismatch of materials
Optical Fiber Core
(8 mm )
SiO2
Cladding
Silicon Waveguide
(0.2 x 0.5 mm2)

13. AIM
Academy Adiabatic Taper
 Waveguide taper length >> wavelength
 In-plane taper: linear, parabolic, exponential,
Gaussian, hyperbolic
 Good Design: 80% power (TE) remains in
source mode
 Novel studies: rectangular taper
 TM mode more stable than TE
E.Marcatili, “Dielectric tapers with curved axes and no loss,” IEEE J.Quantum Electron., QE 21, 307-314 (1985).
G. Jin, S. Shi, A. Sharkawy and D.W. Prather, “Polarization effects in tapered dielectric
waveguides,” Opt. Express, v.11(16), pp.1931-1941 (2003).

14. AIM
Academy Inverted Taper Coupler
 CMOS process flow and fabrication
 100 modal area reduction
 10 coupling efficiency
V.R. Almeida, R.R. Panepucci and M. Lipson, “Nanotaper for compact mode conversion,”
Optics Lett., v.28(15), pp.1302-1304 (2003).
K.K. Lee, L.C. Kimerling, et al., “Mode transformer for miniaturized optical circuits,” Opt.
Lett. v.8(5), pp.498-500 (2005).
T. Tsuchizawa, H. Morita et al., “Microphotonics Devices Based on
Silicon Microfabrication Technology,” IEEE J. Select. Topics Quant. Elect., v.11(1),
pp.232-240 (2005).

15. AIM
Academy Grating Couplers
 Diffract incident light into waveguide
 1D Photonic Crystal (on SOI)
 Couple out-of-plane incident light
into modes propagating away from
in-plane stopband reflector
 Stopband location controlled by etch
depth
 Performance
 TE: 1 dB insertion loss
 35 nm 3dB-bandwidth
Source: http://silicon-photonics.ief.u-psud.fr
C. Li, H. Zhang, M. Yu, G. Q. Lo, Opt. Express 21, 7868-7874 (2013)

16. AIM
Academy 2D Grating Coupler
 2D Photonic Crystal in SOI
 Fiber TE/TM mode couple into different ridge
waveguides
 TM fiber mode transformed into TE
waveguide mode
 Built-in Polarization Diversity
D. Taillaert, H. Chong, P.I. Borel, L.H. Frandsen, R.M. De La Rue and R. Baets, “A Compact Two-Dimensional
Grating Coupler Used as a Polarization Splitter,” IEEE Phot. Tech. Lett., v.15(9), pp.1249-1251 (2003).

17. AIM
Academy Coupler Comparison
Coupler
Type
Coupler
Length/
Area
Coupling
Efficiency
Wavelength
Range
Polarization
Dependent
Adiabatic
Taper
40 μm 80% (TE) ultra-broadband yes
Inverted Taper 100 μm 90 % ultra-broadband yes
1D Grading
2D Grading
~10 μm
~100 μm2
80%
20%
broadband
broadband
yes
no

18. AIM
Academy
Passive Photonics:
Wavelength Division Multiplexing
 Dense WDM function
 Si-compatible, compact footprint
 Microrings, racetracks, slot rings
 Higher-order filters, embedded rings
T. Barwicz et al., Optics Express v.12(7), (2004).

19. AIM
Academy
 Lithography: 248nm
 Q ~ 2000, FSR~16 nm
1st-Order Ring & Racetrack Microring Filters
Q~500
6 mm
6 mmIn
Drop
Silicon
Thru-port 1 2 3 4
Thru-port
D. R. Lim, B. E. Little , K. K. Lee, M. Morse, H. H. Fujimoto, H. A. Haus, and L. C. Kimerling, “Micron-sized channel dropping filters using
silicon waveguide devices,” Proc. SPIE, 3847, pp.65-71 (1999).
Si3N4Si
1520 1540 1560
2mm Ring
FSR=48.6nm; Q=1050
DropPortPower(ArbUnits)
Wavelength (nm)
3 mm Ring
FSR=21nm; Q=3000
5 mm ring
FSR=18nm Q=3875
1x4 WDM (silicon nitride Rings)
1515 1520 1525 1530 1535 1540 1545
Wavelength (nm)
Power--samescale(au)
Port1
Port2
Port3
Port4
Thru
,...2,1
20
=
=
m
r
n
m
eff



20. AIM
Academy
M.A. Popović, H.I. Smith et al., “Multistage high-order microring-resonator add-drop filters,” Opt. Lett., vol. 31,
no. 17, pp. 2571-2573, September 2006.
M.A. Popović, H.I. Smith et al., “High-index-contrast, wide-FSR microring-resonator filter design and realization
with frequency-shift compensation,” in Optical Fiber Communication Conference (OFC/NFOEC) Technical
Digest (Optical Society of America, Washington, DC, March 6-11, 2005), paper OFK1, vol. 5, pp. 213-215.
>2nd Order Rings: Resonance Frequency
 Central ring: different
coupling coefficient 
different resonant frequency
 Compensated ring design
(wider waveguide) ensures
common resonance
frequency
 flatband response
 e-beam lithography
B.E. Little et al., IEEE Photon. Technol. Lett. 16, 2263 (Oct 2004)

21. AIM
Academy WDM Resonator Comparison
Resonator
Type
Quality
Factor
Bandwidth FSR
(L=50 –
100 μm)
Insertion Loss
1st Order Ring 104 - 105
(easy to achieve
high Q)
~103 – 102
GHz
(WDM)
6000 – 3000
GHz
<0.5 dB for gap
< 100 nm
Higher Order
Ring
102 - 103
(high Q
challenging)
~103 – 10
GHz flatband
(DWDM)
6000 – 3000
GHz
>1 dB for gap
< 100 nm
Racetrack 102 - 104 ~103 – 102
GHz
(WDM)
6000 – 3000
GHz
<1 dB for gap
< 100 nm