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EMC-Yoochul-TFTonGlass-edited.ppt - Cornell University


									   Fabrication of Poly-Si TFT
on Flexible Thin Glass Substrate

    Yoochul Jung, Sunghwan Won, D.G. Ast
    (Cornell University, Dep. of Mat. Sci. Eng)

•   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…
                                                                Poly Si
                                                         Barrier layer (SiNx)
                                                     Microsheet™ Glass Wafer
                                                         Barrier layer (SiNx)

                                                                                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)
                                10                                      Vg(V)
                                 5                                      40



    * 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



* 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

  CNF, a National Science Foundation supported National
Nanofabrication Users Network (NNUN) Facility; Corning Inc.

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