motivation for high-speed vcsels...¾testing and characterization top-emitting : 850nm self-aligned...
TRANSCRIPT
1
1Copyright © 2006 by Ahmad N. AL-Omari
“High-Speed Vertical-Cavity Surface-Emitting Lasers with Reduced Electrical and Thermal Constraints on
Modulation Bandwidth”Doctor of Philosophy Final Exam
of
Ahmad N. AL-Omari
Colorado State UniversityElectrical and Computer Engineering Dept.
May 26, 2006
This work has been supported in part by the DARPA under contract number DAAD19-03-1-0059 and by Yarmouk University in Jordan.
2Copyright © 2006 by Ahmad N. AL-Omari
Motivation for high-speed VCSELs
PC mother board
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3Copyright © 2006 by Ahmad N. AL-Omari
Optical connection between individual computers
Motivation for high-speed VCSELs
Optical connection between Printed circuit boards
Chip-to-chipOptical
interconnects
*N. Savage, “Linking with Light,” IEEE Spectrum vol. 39, no. 8, pp 32-36, 2002
Today* IC
VCSEL
PD
VCSEL
PD
waveguide
waveguide
chip A
chip B
4Copyright © 2006 by Ahmad N. AL-Omari
Components level speed limitations
* multimode fiber with a bandwidth distance product 200 MHz-km offers > 100 GHz bandwidth at this distance.
SPEED
40GHz
SPEED
100GHz
Photodiode Wave guide VCSEL(fiber)
SPEED
10GHz
(<1m)*
3
5Copyright © 2006 by Ahmad N. AL-Omari
High-speed VCSEL limitations
Intrinsic limitations (58 GHz* and 71 GHz **)
Extrinsic limitations
- Junction heating- Parasitic circuit effects- Mode control
* K. L. Lear, V.M. Hietala, H.Q. Hou, J. Banas, B.E. Hammons, J. Zolper, and S. Kilcoyne, Advances in Vertical Cavity Surface Emitting Lasers in Trends in Optics and Photonics Series, vol. 15, pp. 69-74, 1997.
** D. Tauber, G. Wang and L. A. Coldren, IEEE Transactions on Electron Devices, vol. 39, pp.2652-2652, November 1992.
Schematic cross section of conventional, top-emitting, high speed VCSEL structure with superposed equivalent
circuit.
6Copyright © 2006 by Ahmad N. AL-Omari
VCSELs fabrication and process development: ► Top-emitting► Bottom-emitting flip-chip bonded
Testing and characterization►Top-emitting : ■ 850nm self-aligned Cu-plated (1st generation)
■ 980nm non–self-aligned Cu-plated(2nd generation)■ 850nm non–self-aligned Cu-plated(2nd generation)
► Bottom-emitting flip-chip bonded
Electrical parasitic analysisEffect of external heating on VCSELs resonance frequency and damping factorCharacterization of spin-on dielectrics Summary (conclusions, achievements and future work)
Outline
4
7Copyright © 2006 by Ahmad N. AL-Omari
Fabrication of top-emitting self-aligned VCSELs.
Top-mirror
Bottom-mirror
Self-aligned top contact
Mesa etching
Wet oxidation
Bottom contact
Isolation
Planarization
Metal wrapping & electroplating
8Copyright © 2006 by Ahmad N. AL-Omari
Self-aligned top contact and mesa etching
The top contact and a smaller diameter photoresistprotecting the aperture served as an etch mask
Annular top contact
Photoresist
etch etch
4μm
1818 photoresist(Pd/Ge/Ti/Pt)
1818 photoresist
(Pd/Ge/Ti/Pt)
Microscopic top view of the (Pd/Ge/Ti/Pt)top contact and a smaller diameter 1818 photoresist
Fabrication of top-emitting self-aligned VCSELs (cont’d)
5
9Copyright © 2006 by Ahmad N. AL-Omari
Mesa sidewall covered with SiNx
APlated Cu
SEM images for a top-emitting VCSEL ready for testing
0μm overlap. 2μm overlap. 4μm overlap.
A’
Plated Cu Ti-Au
p-mirror
n-mirror
GaAs substrate
PolyimideAuGe-Ni-Au
SiNxTi-AuOxide
3 QW
A cross-section of the device along AA’
Fabrication of top-emitting self-aligned VCSELs (cont’d)
10Copyright © 2006 by Ahmad N. AL-Omari
Fabrication of 980nm bottom-emitting flip-chip bonded VCSELs.
Dummy mesa Device mesa Dummy mesa
Substrate
Bottom-mirror
Top-mirror
Topcontact
Ti/Au seed layersemi-insulating GaAs substrate
Plated In
Plated Cu
Bottomcontact
Mesa etching
Wet oxidation
Top contact
Bottom contact
Heat Spreader
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11Copyright © 2006 by Ahmad N. AL-Omari
Fabrication of 980nm bottom-emitting flip-chip bonded VCSELs (cont’d)
Flip-chip bonding
semi-insulating GaAs substrate
Dummy mesa Device mesa Dummy mesa
Substrate
Bottom-mirror
Top-mirror
Plated Cu/In
Mesa foot print
Top contactTi/Au
Bottom contactGe-Au-Ni-Au
Plated Cu/In
SEM images of the device mesas before and after bonding
Ti-AuTopcontact
Bottomcontact
Polyimide
Substrate
Substrate
Substrate
1. The device mesa sidewalls need to be protected from the plated metals2. The ground (Dummy) mesa sidewalls profile needs to be modified.
The incorporation of photosensitive polyimide with the VCSEL die
12Copyright © 2006 by Ahmad N. AL-Omari
Fabrication of 980nm bottom-emitting flip-chip bonded VCSELs (cont’d)
Heat spreader substrateVCSELs die
VCSELs die
Device mesa wrapped with polyimide
Polyimide
Ground mesa wrapped with polyimide and covered with Ti/Au
7
13Copyright © 2006 by Ahmad N. AL-Omari
VCSELs fabrication and process development: ► Top-emitting► Bottom-emitting flip-chip bonded
Testing and characterization►Top-emitting : ■ 850nm self-aligned Cu-plated (1st generation)
■ 980nm non–self-aligned Cu-plated(2nd generation)■ 850nm non–self-aligned Cu-plated(2nd generation)
► Bottom-emitting flip-chip bonded
Electrical parasitic analysisEffect of external heating on VCSELs resonance frequency and damping factorCharacterization of spin-on dielectrics Summary (conclusions, achievements and future work)
Outline
14Copyright © 2006 by Ahmad N. AL-Omari
0
0.2
0.4
0.6
0.8
1
0 5 10I bias (mA)
Pout
(mW
)
10μm oxide apertureIth~0.8mA
Cu-plated Au-wrappedPolyimide-wrapped*
CW L-I characteristics
* A. N. AL-Omari and K. L. Lear, “Polyimide-Planarized Vertical-Cavity Surface Emitting Lasers with 17.0 GHz Bandwidth,” IEEE PTL, vol. 16, 969-971, 2004.
Top-emitting 850nm VCSELs with a self-aligned contact and Cu-plated heatsink (1st generation)
0.91 mW
0.76 mW
0.66mW
0
1
2
3
0 5 10 15 20Active diameter(μm)
Rth
(°C
/mW
)
[*]Polyimide-wrappedAu-wrappedCu-plated
VCSELs thermal resistance vs. active diameter.
Polyimide-wrapped* Au-wrapped Cu-platedPout↑ by 15% Pout↑ by 38%
Rth↓ by 25% Rth↓ by 44%
f3dB ↑ by 12%
VCSELs modulation response vs. bias currents.
0
4
8
12
16
0 2 4 6 8Current (mA)
f3dB
(GH
z)
Cu-plated f3dBmax =16.3GHzPolyimide-wrapped*f3dBmax =14.6GHz
Plated Cu
SEM photo for a completed Au-wrapped high-speed VCSEL with 18μm diameter and 10μm oxide aperture.
SEM photo for a completed Cu-electroplated high-speed VCSEL with 18mm diameter and 10mm oxide aperture.
G
S
G
G
S
G
Au-wrapped
Polyimide
Polyimide
8
15Copyright © 2006 by Ahmad N. AL-Omari
0
0.2
0.4
0.6
0.8
1
0 5 10I bias (mA)
Pout
(mW
)
10μm oxide apertureIth~0.8mA
Cu-plated Au-wrappedPolyimide-wrapped*
CW L-I characteristics
* A. N. AL-Omari and K. L. Lear, “Polyimide-Planarized Vertical-Cavity Surface Emitting Lasers with 17.0 GHz Bandwidth,” IEEE PTL, vol. 16, 969-971, 2004.
Top-emitting 850nm VCSELs with a self-aligned contact and Cu-plated heatsink (1st generation)
0.91 mW
0.76 mW
0.66mW
0
1
2
3
0 5 10 15 20Active diameter(μm)
Rth
(°C
/mW
)
[*]Polyimide-wrappedAu-wrappedCu-plated
VCSELs thermal resistance vs. active diameter.
Polyimide-wrapped* Au-wrapped Cu-platedPout↑ by 15% Pout↑ by 38%
Rth↓ by 25% Rth↓ by 44%
f3dB ↑ by 12%
VCSELs modulation response vs. bias currents.
0
4
8
12
16
0 2 4 6 8Current (mA)
f3dB
(GH
z)
Cu-plated f3dBmax =16.3GHzPolyimide-wrapped*f3dBmax =14.6GHz
16Copyright © 2006 by Ahmad N. AL-Omari
1st generation vs. 2nd generation
► Cu-plated quality ■ Density (current density,
duty cycle, temperature, sample orientation)
■ Adhesion (oxide layer, seed layer)
► Signal pad open circuit (shrinkage of the polyimide during the curing process)
► Relief of device dimensions (yield)
X2,000 X2,200
1st generation 2nd generation
9
17Copyright © 2006 by Ahmad N. AL-Omari
Top-emitting 850nm VCSELs with a non-self-aligned contact and Cu-plated heatsink (2nd generation)
Ground
Polyimide
Signal
Polyimide
Ground
Plated Cu
SEM images of (a) a 26μm mesa diameter including coplanar waveguide probe pads
plated with ~3μm of Cu.
2μm overlap.
0μm overlap.
18Copyright © 2006 by Ahmad N. AL-Omari
Top-emitting 850nm VCSELs with a non-self-aligned contact and Cu-plated heatsink (2nd generation)
Ground
Polyimide
Signal
Polyimide
Ground
Plated Cu
SEM images of (a) a 26μm mesa diameter including coplanar waveguide probe pads
plated with ~3μm of Cu.
2μm overlap.
0μm overlap.
f3dB vs. (I-Ith)1/2
0
5
10
15
20
0 1 2 3(I-Ith)1/2mA1/2
f 3dB
(GH
z)
10μm/2μm/8.1(GHz/mA1/2)12μm/2μm/5.7(GHz/mA1/2)18μm/2μm/4.0(GHz/mA1/2)
Legend key: Active area/overlap/MCEFStraight lines: fit to f3dB=MCEF(I-Ith)1/2
8μm/2μm/15.6(GHz/mA1/2)8μm/0μm/11.8(GHz/mA1/2)
+
f3dB ↑ by 11% compared tof3dB ~ 18 GHz at 8kA/cm2
+
+
0
1
2
3
0 5 10 15 20Active diameter(μm)
Rth
(°C
/mW
)
Rth vs. active diameter.
Polyimide-wrappedAu-wrappedCu-plated (1st generation)Cu-plated(2nd generation)
■ Rth ↓ by 57%, 52%, and 45% compared to , ,and respectively
10
19Copyright © 2006 by Ahmad N. AL-Omari
Top-emitting 850nm VCSELs with a non-self-aligned contact and Cu-plated heatsink (2nd generation) (cont’d)
0
5
10
15
20
0 1 2 3(I-Ith)1/2mA1/2
f3dB
(GH
z)
f3dB vs. (I-Ith)1/2
8μm/2μm/15.6(GHz/mA1/2)Active area/overlap/MCEF
843 845 847 849λ(nm)
I bias (Ibias-Ith)1/2
1mA 0.71mA1/2
2mA 1.22mA1/2
3mA 1.58mA1/2
4mA 1.87mA1/2
5mA 2.21mA1/2
Wavelength spectrum for an 8μm active diameter VCSEL with a 3μm of plated Cu at different bias currents.
20Copyright © 2006 by Ahmad N. AL-Omari
SEM images for a top-emitting VCSEL ready for testing
Mesa sidewall covered with SiNx
Plated Cu
0μm overlap.
4μm overlap.
Non-self-aligned top-emitting Cu-plated 980nm VCSELs (2nd generation)
0
1
2
3
4
5
6
0 10 20 30 40Current (mA)
Pow
er(m
W)
0
1
2
3
4
5
6
Vol
tage
(V)
0um overlap4um overlap
CW L-I-V characteristics for a 26mm mesa diameter VCSEL with a
9μm oxide-aperture diameter.
power output ↑ by 131%
11
21Copyright © 2006 by Ahmad N. AL-Omari
SEM images for a top-emitting VCSEL ready for testing
Mesa sidewall covered with SiNx
Plated Cu
0μm overlap.
4μm overlap.
Non-self-aligned top-emitting Cu-plated 980nm VCSELs (2nd generation)
0
1
2
3
4
5
6
0 10 20 30 40Current (mA)
Pow
er(m
W)
0
1
2
3
4
5
6
Vol
tage
(V)
0um overlap4um overlap
CW L-I-V characteristics for a 26mm mesa diameter VCSEL with a
9μm oxide-aperture diameter.
power output ↑ by 131%
0
1
2
3
0 5 10 15 20Active diameter(μm)
Rth
(°C
/mW
)
Cu-plated 850nm (2nd)Cu-plated 980nm (2nd )
Polyimide-wrappedAu-wrappedCu-plated 850nm (1st)
Rth vs. active diameter.
Rth↓ by 50%f3dB ↑ by 30 and 40% for and ,respectively, compared tof3dB max ~ 9.8 GHz at 8.9kA/cm2
0
2
4
6
8
10
12
0 1 2 3 4
(I-Ith)1/2 (mA)1/2
f 3dB
(GH
z)0um overlap2um overlap4um overlap
f3dB vs. (I-Ith)1/2
5.0, 3.0, and 2.2 GHz/mA1/2MCEF
f3dB=MCEF(I-Ith)1/2
22Copyright © 2006 by Ahmad N. AL-Omari
0
1
2
3
0 5 10 15 20Active diameter(μm)
Rth
(°C
/mW
)
Previous workPrevious work/Bottom-emittingPresent work/1st generationPresent work/1st generationPresent work/2nd generationPresent work/2nd generationPresent work/2nd generation
VCSELs’ thermal resistance vs. active diameter
aRth ⋅
=ξ41
fit
12
23Copyright © 2006 by Ahmad N. AL-Omari
VCSELs bandwidths vs. current densities
0
5
10
15
20
0 10 20 30 40 50
Jbias(kA/cm2)
f 3dB
(GH
z)
work by othersprevious work by authorpresent workbenchmark for reliability
Calculated
Photonic crystal
24Copyright © 2006 by Ahmad N. AL-Omari
Bottom-emitting flip-chip bonded 980nm VCSELs
CW L-I characteristics for 980nm bottom-emitting VCSELs before and after flip-chip bonding.
Heat spreader substrateVCSELs die
polyimide- wrapped device mesa
VCSELs dieG
G
0
5
10
15
20
25
0 5 10 15 20 25 30 35
Current(mA)
Pow
er(m
W)
0
5
10
15
20
25
0 5 10 15 20 25 30 35
Current(mA)
Pow
er(m
W)
Before bonding: doted After bonding: solid
10μm
14μm
16μm
20μm
The flip-chip bonding↑ the maximum output power for VCSELs with active diameters of 10, 14, 16, and 20μm by 10, 11, 19, and 25%, respectively.
13
25Copyright © 2006 by Ahmad N. AL-Omari
VCSELs fabrication and process development: ► Top-emitting► Bottom-emitting flip-chip bonded
Testing and characterization►Top-emitting : ■ 850nm self-aligned Cu-plated (1st generation)
■ 980nm non–self-aligned Cu-plated(2nd generation)■ 850nm non–self-aligned Cu-plated(2nd generation)
► Bottom-emitting flip-chip bonded
Electrical parasitic analysisEffect of external heating on VCSELs resonance frequency and damping factorCharacterization of spin-on dielectrics Summary (conclusions, achievements and future work)
Outline
26Copyright © 2006 by Ahmad N. AL-Omari
Electrical parasitic analysis
Top-emitting long oxide VCSEL Cm = 142.4 fFCp = 56.9 fF
Top-emitting short oxide VCSEL
Cm = 70 fF (~49%)Cp = 21 fF (37%)
Estimated values for a 7μm oxide aperture:
High speed VCSEL cross section
Polyimide
Substrate
Cp Co2 Metal pad
Co1 Ca
Oxide
QWs
7μm
14
27Copyright © 2006 by Ahmad N. AL-Omari
Electrical parasitic analysis
CwrapH+CwrapV ~ 236fFf3dB ~15.6 GHz (simulated) f3dB ~16.3 GHz (measured)
Polyimide
Gold rap(3000Å)
Ti/Au
Si3N4 (1000Å)
Cwrap V
Cwrap H 0
0.2
0.4
0.6
0.1 1 10 100Frequency (GHz)
Vou
t(V
)
6μm extension of Cu around the mesa2μm extension of Cu around the mesa
Electrical parasitic analysis (cont’d)
PSpice simulations pointed out that purely electrical circuit effects would allow a maximum 3dB bandwidth of
about 63GHz
It is believed that Cu-plated VCSELs with 6μm extension of Cu around the mesa are limited by electrical parasitics rather than thermal effects.
28Copyright © 2006 by Ahmad N. AL-Omari
VCSELs fabrication and process development: ► Top-emitting► Bottom-emitting flip-chip bonded
Testing and characterization►Top-emitting : ■ 850nm self-aligned Cu-plated (1st generation)
■ 980nm non–self-aligned Cu-plated(2nd generation)■ 850nm non–self-aligned Cu-plated(2nd generation)
► Bottom-emitting flip-chip bonded
Electrical parasitic analysisEffect of external heating on VCSELs resonance frequency and damping factorCharacterization of spin-on dielectrics Summary (conclusions, achievements and future work)
Outline
15
29Copyright © 2006 by Ahmad N. AL-Omari
Effect of external heating on VCSELs resonance frequency and damping factor
0
0.4
0.8
1.2
1.6
0 1 2 3 4 5 6 Current(mA)
Pow
er(m
W)
974 975 976λ (nm)
Bias point for RIN measurements
18μm mesa diameter/8μm active diameter
L-I of an 18μm mesa diameter with an 8μm active diameter. The inset shows the wavelength spectrum at which the relative intensity noise (RIN) was measured.
Hp 8569B spectrum analyzer 23
dB34dB
Amplifiers
Temp. controllerILX-LDT-5910B
Photodiode
RIN measurement setup.
Keithley source meter
Temp. controller stage
30Copyright © 2006 by Ahmad N. AL-Omari
0
2
4
6
8
0 10 20 30 40 50 60Stage temperature (°C)
f r(G
Hz)
0
5
9
14
18
Γx1
09 (Rad
/s)
Effect of external heating on VCSELs resonance frequency and damping factor (cont’d)
Relaxation oscillation frequency and damping as a function of the stage
temperature.
4 6 8Frequency(GHz)
a.u.
10C20C30C40C50C
RIN spectra of the investigated VCSEL under different stage
temperatures.
16
31Copyright © 2006 by Ahmad N. AL-Omari
0
4
8
12
16
0 10 20 30 40 50
fr2(GHz)2
Γx1
09 (Rad
/s)
2~ rKfΓ
Temperature increase in 10°C step
60°C
10°C
Effect of external heating on VCSELs resonance frequency and damping factor (cont’d)
Slope=K=0.36ns
Kf dB )22(3 π=
A maximum intrinsic 3dB frequency bandwidth of 24.7GHz is estimated using:
Damping vs. the relaxation oscillation frequency squared at different stage temperatures
32Copyright © 2006 by Ahmad N. AL-Omari
VCSELs fabrication and process development: ► Top-emitting► Bottom-emitting flip-chip bonded
Testing and characterization►Top-emitting : ■ 850nm self-aligned Cu-plated (1st generation)
■ 980nm non–self-aligned Cu-plated(2nd generation)■ 850nm non–self-aligned Cu-plated(2nd generation)
► Bottom-emitting flip-chip bonded
Electrical parasitic analysisEffect of external heating on VCSELs resonance frequency and damping factorCharacterization of spin-on dielectrics Summary (conclusions, achievements and future work)
Outline
17
33Copyright © 2006 by Ahmad N. AL-Omari
Dielectric130μm
GaAs substrate
Signal contactTi-Au (0.33 μm)
Ground contactTi-Au-Ti (0.37 μm)
Ti-Au (0.33 μm)
Capacitor cross-section at AA’
Characterization of spin-on dielectrics
Metal padsTi/Au
G
S
G
Ti/Au/Ti
Dielectric
A’
A
SEM photo for a completed capacitor ready for testing.
Futurrex, Inc. Dow Chemical Cyclotene™HD Microsystems™HD Microsystems™Manufacturer
SOG IC1-200BCB-4024PI-2723HD-8000Specific variety
►Capacitors from the different dielectric materials with various thicknesses were fabricated.► Microwave scattering parameters were measured.
34Copyright © 2006 by Ahmad N. AL-Omari
Characterization of spin-on dielectrics (cont’d)
1
2
3
4
5
0 10 20 30 40Frequency (GHz)
ε'
PI-2723
HD-8000 BCB-4024
SOG IC1-200
ε’ vs. freq.
40
50
60
70
80
90
100
110
0 10 20 30 40Frequency (GHz)
Cp(f
F)
BCB-4024 IC1-200 (÷4)
HD-8000
PI-2723
MeasuredFit
Capacitance vs. frequency.
0
6
12
18
24
0 2 4 6 8Dielectric thickness (μm)
Rlo
ss ( Ω
)
HD 8000PI 2723SOG IC1 200BCB 4024
Dielectric loss equivalent resistance vs. dielectric thickness.
Dielectric permitivity as a function of frequency was extracted and is well approximated by a second degree polynomial.A circuit model for the pad capacitance is developed based on geometrical and physical considerations
18
35Copyright © 2006 by Ahmad N. AL-Omari
VCSELs fabrication and process development: ► Top-emitting► Bottom-emitting flip-chip bonded
Testing and characterization►Top-emitting : ■ 850nm self-aligned Cu-plated (1st generation)
■ 980nm non–self-aligned Cu-plated(2nd generation)■ 850nm non–self-aligned Cu-plated(2nd generation)
► Bottom-emitting flip-chip bonded
Electrical parasitic analysisEffect of external heating on VCSELs resonance frequency and damping factorCharacterization of spin-on dielectrics Summary (conclusions, achievements and future work)
Outline
36Copyright © 2006 by Ahmad N. AL-Omari
AchievementsWe have fabricated and characterized high-speed oxide-confined VCSELs with high-
speed performance.
A mask set was designed with flexibility to investigate various VCSEL structures. A complete fabrication process for self- and non-self-aligned VCSELs was developed.
Self- and non-self-aligned top-emitting 850 and 980nm high-speed VCSELs were fabricated and characterized. The effect of a heat-sinking layer on the VCSELsperformance was demonstrated.
↑ 40%↑ 131%↓ 50%↓ 63%2ndNon-self-aligned 980 nm
↑ 11%↑ 42%↓ 57%↓ 63%2ndNon-self-aligned 850nm
↑ 12%↑ 38%↓ 44%↓ 63%1stSelf-aligned 850nm
Modulation bandwidth
Output power
Thermal resistance
Padcapacitance
GenerationTop-emitting
↓↓ Preferred trend
Un-preferred trend
19
37Copyright © 2006 by Ahmad N. AL-Omari
Achievements (cont’d)
Bottom-emitting non-self-aligned flip-chip bonded 980nm devices were fabricated and characterized. ■ Flip-chip bonding ↑ the output power by up to 25%.■ The lasers exhibited a slope and wall plug efficiencies of 71% and 47%, respectively.
Increasing the stage temperature tends to reduce both the resonance frequency and the damping factor, hence limiting the modulation bandwidth of the VCSEL. The maximum intrinsic 3dB frequency bandwidth was estimated to be 24.7GHz.
The effect of annealing, etching, and oxidation on the Ti-Pd-Ti-Au-Ti-Pd and Ti-Ni-Ti-Ni metal systems specific contact resistance was investigated. The later metal system exhibited a lower specific contact resistance relative to the former one.
Dielectric properties of 4 different spin-on dielectrics were investigated. A circuit model for the pad capacitance was obtained.
38Copyright © 2006 by Ahmad N. AL-Omari
Achievements (cont’d)
Development of ► a computer controlled planetary stage for metal step coverage.► an In-situ monitor for end point etch detection.► a more accurate sample temperature monitoring system during the oxidation process with manual feedback.► an electrically-heated stage to replace the flip-chip bonding gas-heated stage for more accurate and repeatable bonding process.
Technical support for almost all of the cleanroom equipments including writing new and editing exist standard equipment procedures, maintenance, modifications needed to carry out specific processes, and fixing.
Trained more than 20 cleanroom users (EE, CH, C, and PH) on various equipments and helped them accomplish their goals ( mask design, process development, and fabrication issues).
20
39Copyright © 2006 by Ahmad N. AL-Omari
Conclusions► Scaling down mesa diameter and active area reduce the VCSEL self-heating and mesa
parasitic capacitance.
► The effect of heat-sinking on the performance of high-speed VCSELs was demonstrated at 850 and 980nm. The presence of heat-spreading layers the thermal impedance, the maximum output power, and the f3dB modulation bandwidth.
► There is a tradeoff between the heat-spreading layer overlap for thermal impedance reduction and the resulting parasitic capacitance for high-speed operation.
► The flip-chip bonding of bottom-emitting devices demonstrated its effect on the VCSEL performance.
► The thermal resistance measurements indicated the importance of lateral heat flow to the mesa sidewalls.
► The reduction in the resonance frequency and damping factor due to the increase in stage temperature is believed to cause the VCSEL’s modulation bandwidth to saturate.
► The use of photosensitive HD-8000 polyimide gave reliable and repeatable results.
40Copyright © 2006 by Ahmad N. AL-Omari
Future workMore fabrication runs need to be done to optimize the VCSEL fabrication process.
Investigation of the high threshold voltage values and nonlinear I-V characteristics that the fabricated devices suffered from.
At which thickness will further increase in the heat-sinking layer cease to have considerable effect on the VCSEL thermal resistance reduction?
Other materials with better characteristics such as CVD diamond can be used to replace SiNx and polyimide.
To improve the heat sinking for the current bottom-emitting flip-chip devices, the device mesa sidewalls need to be plated with a heat sinking layer.
Epitaxial structure optimization since tradeoffs exists between different factors affecting high-speed operation.
A comparison between the measured and modeled performance need to be done to develop further understand of the major issues affecting VCSEL speed and model modifications that results in more accurate predictions.
21
41Copyright © 2006 by Ahmad N. AL-Omari
ACKNOWLEDGMENTS
The technical assistance from G. Dang and the support of R. Athale, W. Chang, and G. Simonis.
42Copyright © 2006 by Ahmad N. AL-Omari
JOURNALS[1] A. N. AL-Omari and K. L. Lear, “Polyimide-Planarized Vertical-Cavity Surface Emitting Lasers with 17.0 GHz
Bandwidth,” IEEE Photonics Technology Letters, Vol. 16, No. 4, pp. 969-971, April 2004.[2] A. N. AL-Omari and K. L. Lear, “VCSELs with a Self-Aligned Contact and Copper-Plated Heatsink,” IEEE Photonics
Technology Letters, Vol. 17, No. 9, pp. 1767- 1769, September 2005. [3] A. N. AL-Omari and K. L. Lear, “Dielectric Characteristics of Spin-Coated Dielectric Films Using On-Wafer Parallel-
Plate Capacitors at Microwave Frequencies,” IEEE Transactions on Dielectrics and Electrical Insulation. Vol. 12, No. 6, pp. 1151- 1161, December 2005.
[4] A. N. AL-Omari et al., “Low Thermal Resistance, High Speed, Top Emitting 980nm VCSELs,” IEEE Photonics Technology Letters, Vol. 18, No.11, pp. 1225- 1227, November 2006.
[5] A. N. AL-Omari and K. L. Lear, “Low Current Density, Inverted Polarity, 850nm VCSELs with 18GHz Bandwidth,”IEEE Photonics Technology Letters, in preparation.
CONFERENCES[1] A. N. AL-Omari and K. L. Lear, “High-Speed Polyimide-Planarized, Vertical-Cavity Surface Emitting Lasers,”
Proceedings of SPIE, Vertical-Cavity Surface-Emitting Lasers VIII Conf., Vol. 5364, pp.73-79, San Jose, CA, June 2004.
[2] A. N. AL-Omari, Stewart. A. Feld, and K. L. Lear, “Bottom-Emitting, Flip-Chip VCSEL Structure with Reduced Constraints on Modulation Response,” Proceedings of SPIE, Photonic Devices and Algorithms for Computing VI Conf.,Vol. 5556, pp.199-205, Denver, CO, August 2004.
[3] A. N. Al-Omari, S. A. Feld, and K. L. Lear, “Novel VCSEL Structure with Reduced Constraints on Modulation Bandwidth,” Proceedings of the 17th Annual Meeting of the IEEE - LEOS 2004, Vol.1, pp.328-329, Rio Grande, Puerto Rico, 7-11 November 2004.
[4] A. N. Al-Omari and K. L. Lear, “Improved Performance of Top-Emitting, Oxide-Confined VCSELs with Self-Aligned Contact and Cu-Plated Heatsink,” IEEE Conference on Quantum Electronics and Laser Science, Vol. 2, pp. 1020-1022, Baltimore, Maryland, 22–27 May 2005.
Publications
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43Copyright © 2006 by Ahmad N. AL-Omari
[5] A. N.AL-Omari et al.,“Low Thermal Resistance, Low Current Density, High-Speed 980 and 850nm VCSELs,” 20th IEEE International Semiconductor Laser Conference, Waikoloa, Hawaii, 17–21 September 2006. (submitted)
OTHER PRESENTATIONS[1] A. N. AL-Omari and K. L. Lear, “High-Speed Polyimide-Planarized, 850nm VCSELs with 17.0 GHz Bandwidth,”
Colorado Photonics Industry Association 4th annual meeting, Boulder, CO, November 20, 2003. [2] A. N. AL-Omari and K. L. Lear “17 GHz polyimide planarized VCSELs” seminar at Cielo Communication Inc.,
Broomfield, CO, March 2004. [3] K.L. Lear and A. N. AL-Omari, “VCSEL drives higher-bandwidth LANs,” Fiber Systems Europe in association
with LIGHTWAVE Europe (FSE&LE), p.7, United Kingdom, May 2004. (Editor: Tami Freeman)[4] A. N. AL-Omari and K. L. Lear, “17 GHz polyimide planarized VCSELs for chip-to-chip optical interconnects,”
poster presented at Information Science and Technology Colloquium, Fort Collins, CO, April 13-14, 2005. [5] A. N. Al-Omari et al., “Thermal impact and management for high-speed VCSELs,” poster presented at Information
Science and Technology Colloquium, Fort Collins, CO, April 13-14, 2005. [6] A. N. AL-Omari, K. L. Lear, and C. Menoni, “Laboratory Manual for Electrical Engineering Fundamentals
(EE192),” Colorado State University, Electrical and Computer Engineering Department, 2000.
Publications (cont’d)
Memberships and societies