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Fabrication of Poly-Si TFT on Flexible Thin Glass Substrate
Yoochul Jung, Sunghwan Won, D.G. Ast
(Cornell University, Dep. of Mat. Sci. Eng)
2006.06.28
Outline
1. Motivation
2. Comparison of Polymer and Glass substrates.
3. Processing of Pocket Fabrication
4. Characteristics of poly-Si TFT on Flexible Glass Substrate
5. Summary and Discussion
Ast Group
Motivation – Development of “Displays”
CRT TFT (a-Si) -LCD TFT (poly-Si) -LCD
Advantages of flexible displayLess apt to break, Roll-up, Less weight and volume
Flexible Displays are being developed as the next generation displays
Taken from Philips Inc.
Ast Group
Polymer Based Display Vs. Glass Based Display
Large CTE (PET, 65 x 10-6/°C) Low and adaptable CTE (Si, 2 x 10-6/°C)
xx surface finish to less compatible material
High surface finish
α-Si:H
Compatible material with
Low temperature oxide
α-Si:H
Max. processing temperature ~ 300 °C Max. processing temperature ~ 600 °C
Polymer substrates Glass substrates
Ast Group
Laser recrystallized Si with barrier layers CVD poly-Si
MILC silicon
Laser recrystallized Si (no thermal barrier)
Mismatched with Si, thermal stress
LPCVD Poly-Si TFT on MS Glass
* Microsheet borosilicate glass contains boron* Boron acts as p-dopant in Si* Boron may migrate into Si-electronics during poly-deposition* Barrier layer is required1
* Mechanical support is required to handle Microsheet glass2
For 1, SiNX, LTO layer used for barrier layer For 2, Special support needs to be designed…
Ast Group
Microsheet™ Glass WaferBarrier layer (SiNx)
Poly Si
Gate
DrainSource
Barrier layer (SiNx)
Gate
SiO2
SiO2
Si-Framed Pocket Fabrication
Ast Group
MS Glass substrate
Real photo will be added here…. 1. No bonding between glass and Si piece rails2. Free expansion and shrinkage3. Controlling capillary phenomena
Fabrication Process of Si-Framed Pocket
Si
Pyrex Spacer (~ 500 m)
Bottom of EV 501 Bonder Chamber
Graphite chuck
120 N, 350 °C, 1000 V
Positive bias
Negative bias
Si pieces (~ 300 m)
Bottom of EV 501 Bonder Chamber
Positive bias
Si
Ast Group
Drain Voltage (V)-10 0 10 20 30 40
I D C
urre
nt (A
)
0.0
2.0e-5
4.0e-5
6.0e-5
8.0e-5
1.0e-4
1.2e-4
Gate Voltage (V)-20 0 20 40 60 80
Log
I D C
urre
nt (A
)
1e-10
1e-9
1e-8
1e-7
1e-6
1e-5
1e-4
1e-3 Vg(V)40
302010
Vd(V)10 5
0.1
Base line Characteristics (TFT on Si Wafer, Thermal Anneal)
Ast Group
* W/L = 55um/8um* Channel Mobility 7 cm2/Vs
* Poly-Si active layer: 620°C, 100nm* Gate oxide (LTO): 400°C, 100nm
LPCVD poly-Si TFT on the Glass
Ast Group
Gate Voltage (V)-20 -10 0 10 20 30 40 50
I D C
urre
nt (A
)
1e-9
1e-8
1e-7
1e-6
1e-5
1e-4
* Poly-Si active layer: 550°C, 100nm* Gate oxide (LTO): 400°C, 100nm
* TFT was short after thermal anneal* 580C poly-Si active layer* 620C, 24 hrs* SIMS data
Ast Group
SIMS Analysis
1.E+16
1.E+17
1.E+18
1.E+19
1.E+20
1.E+21
1.E+22
0 50 100 150 200 250 300 350 400
Depth [nm]
B [a
t/cm
3]
1.E+04
1.E+05
1.E+06
1.E+07
Si [c
t/sec
]
p-Si SiO2 SiN Glass
Boron
Si
* After 620°C, 24 hrs anneal Boron diffused out from the glass !* CTE mismatch caused thermal stress* Laser anneal was done instead of the conventional thermal anneal
XRD of Poly-Silicon (Thermal, Laser Anneal)
620C Poly-Si on oxide on Si
2 Theta
20 25 30 35 40 45 50
Inte
nsity
(A.U
.)
(111) (220)
580C Poly-Si on Oxide on Si
2 Theta
20 25 30 35 40 45
Inte
nsity
(A.U
)
(111)
20 25 30 35 40 45 50
Inte
nsity
(A.U
)
500C poly-Si on Glass 500C on Glass after Laser Anneal at 283 mJ
Ast Group
Characteristics (TFT on MS Glass, Laser Anneal)
Ast Group
Summary and Future Plan
1. Fixture developed to process 2. Base 3. CVD poly 4. Laser recrystallized 5. According to SIMS analysis, boron diffused into poly-Si layer after thermal annealing of 620C, 24 hrs
* Future Plan *1. Better effective Hydrogenation2. Improvement of characteristics by Recrystallization - Rapid Thermal Anneal (or standard anneal) - Ni catalyzed crystallization3. Stress and bending test 4. Bending
Ast Group
Acknowledgement
CNF, a National Science Foundation supported National Nanofabrication Users Network (NNUN) Facility; Corning Inc.