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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 • Motivation • 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 Taken from Philips Inc. Advantages of flexible display Less apt to break, Roll-up, Less weight and volume Flexible Displays are being developed as the next generation displays Ast Group Polymer Based Display Vs. Glass Based Display Polymer substrates Glass substrates Max. processing temperature ~ 300 °C Max. processing temperature ~ 600 °C Large CTE (PET, 65 x 10-6/°C) Low and adaptable CTE (Si, 2 x 10-6/°C) Mismatched with Si, thermal stress High surface finish xx surface finish to less compatible material Compatible material with α-Si:H α-Si:H Laser recrystallized Si with barrier layers CVD poly-Si Low temperature oxide MILC silicon Laser recrystallized Si (no thermal barrier) Ast Group 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 Source Gate Drain For 2, Special support needs to be designed… Gate Poly Si SiO2 Barrier layer (SiNx) Microsheet™ Glass Wafer Barrier layer (SiNx) SiO2 Ast Group Si-Framed Pocket Fabrication MS Glass substrate Real photo will be added here…. • No bonding between glass and Si piece rails • Free expansion and shrinkage • Controlling capillary phenomena Ast Group Fabrication Process of Si-Framed Pocket 120 N, 350 °C, 1000 V Negative bias Pyrex Spacer (~ 500 mm) Graphite chuck Bottom of EV 501 Bonder Chamber Si pieces (~ 300 mm) Positive bias Si Si Bottom of EV 501 Bonder Chamber Positive bias Ast Group Base line Characteristics (TFT on Si Wafer, Thermal Anneal) Vd(V) 10 Vg(V) 5 40 0.1 30 20 10 * W/L = 55um/8um * Poly-Si active layer: 620°C, 100nm * Channel Mobility » 7 cm2/Vs * Gate oxide (LTO): 400°C, 100nm Ast Group LPCVD poly-Si TFT on the Glass * TFT was short after thermal anneal * 580C poly-Si active layer * 620C, 24 hrs * SIMS data * Poly-Si active layer: 550°C, 100nm * Gate oxide (LTO): 400°C, 100nm Ast Group SIMS Analysis Si Boron * 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 Ast Group XRD of Poly-Silicon (Thermal, Laser Anneal) (111) (111) (220) 500°C poly-Si on Glass 500°C 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 * • Better effective Hydrogenation • Improvement of characteristics by Recrystallization - Rapid Thermal Anneal (or standard anneal) - Ni catalyzed crystallization 3. Stress and bending test 4. Bending Ast Group Acknowledgement CNF, a National Science Foundation supported National Nanofabrication Users Network (NNUN) Facility; Corning Inc.
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