Fabrication of Poly-Si TFT on Flexible Thin Glass Substrate Yoochul Jung, Sunghwan Won, D.G. Ast (Cornell University, Dep. of Mat. Sci. Eng)

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 display Less 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 /°C) Low and adaptable CTE (Si, 2 x /°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 °CMax. processing temperature ~ 600 °C Polymer substrates Glass substrates Ast Group Laser recrystallized Si with barrier layersCVD 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 required 1 * Mechanical support is required to handle Microsheet glass 2 For 1, SiN X, LTO layer used for barrier layer For 2, Special support needs to be designed… Ast Group Microsheet™ Glass Wafer Barrier layer (SiN x ) Poly Si Gate DrainSource Barrier layer (SiN x ) Gate SiO 2

Si-Framed Pocket Fabrication Ast Group MS Glass substrate Real photo will be added here…. 1.No bonding between glass and Si piece rails 2.Free expansion and shrinkage 3.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

Vg(V) Vd(V) Base line Characteristics (TFT on Si Wafer, Thermal Anneal) Ast Group * W/L = 55um/8um * Channel Mobility  7 cm 2 /Vs * Poly-Si active layer: 620 ° C, 100nm * Gate oxide (LTO): 400 ° C, 100nm

LPCVD poly-Si TFT on the Glass Ast Group * 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 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) (111) (220) (111) 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 * 1.Better effective Hydrogenation 2.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.