Nanocrystalline Silicon Thin Film Transistors on Optically Clear Polymer Foil Substrates
- PDF / 320,709 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 66 Downloads / 202 Views
H2.7.1
Nanocrystalline Silicon Thin Film Transistors on Optically Clear Polymer Foil Substrates Alex Kattamis, I-Chun Cheng, Ke Long, James C. Sturm, Sigurd Wagner Abstract We have fabricated TFTs of nanocrystalline silicon (nc-Si) at 150°C on clear polymer substrates (coefficients of thermal expansion, α~45 to 55ppm/K), on Kapton® 200E (α=17ppm/K), and on Corning 1737 glass (α=3ppm/K) for comparison. Because thermally stable polymers, such as Kapton® 200E polyimide, have glass transition temperatures as high as 325°C, they are candidates for direct substitution of display glass. The stresses developed in the substrate and device layers, due to α, are reduced by decreasing the thickness of the active layers, by cutting the layers into islands separated by exposed substrate, and by designing stresses, via plasma conditions, into the SiNx passivating layers. By using these three techniques we have made nc-Si TFTs on high Tg, and high α, clear polymer foils with electron mobilities of up to 18cm2/Vs. When integrated with bottom-emitting organic light emitting diodes, such devices will allow for a 10x reduction in pixel TFT areas, compared to TFTs of amorphous silicon. Introduction Flexible displays are the next technology generation for flat-panel displays. Therefore interest is growing in building high-performance TFT backplanes on flexible substrates. Flexible polymer foils are attractive for large area displays, because they are lightweight and allow for the possibility of roll-able displays. Ideally, a flexible backplane will be made using fabrication processes that already have been developed for glass with only minor changes. Nanocrystalline silicon (nc-Si) TFTs are capable of both n- and p-type operation, have at least ten times the nchannel ON current of amorphous silicon (a-Si) TFTs, and can be plasma deposited at temperatures upwards of 150°C. This makes them CMOS capable and good candidates for pixel circuits in active-matrix organic light emitting diode (AMOLED) displays. There is growing interest in AMOLED displays because active-matrix addressing allows for high pixel count and long OLED life [1]. For AMOLED displays with TFT pixel drivers and bottom-emitting OLEDs mounted on the same plane, the substrate must be optically clear. When polymer substrates are heated that high though, the thermal expansion mismatch between silicon TFT materials and the polymer substrate becomes a challenge. The effects of mismatch become more pronounced as process temperature is increased and this mismatch, and built-in stresses in the device films, can combine to fracture TFT structures during processing. Today pixel circuits for AMOLED displays on polymer substrates are conceived with hydrogenated amorphous-silicon thin film transistors (a-Si TFTs) [2,3]. a-Si TFTs have been fabricated at temperature as low as ~100ºC [4] and have been produced on clear polymers such as poly-ethylene terephthalate (PET) [5]. However since PET has a glass transition temperature (Tg) >αfilm, as the substrate cools from the deposition tempera
Data Loading...
