Key technical issues of flexible stainless steel foil substrates are addressed for OLED display backplane
applications. Surface roughness and corresponding planarization layer technology development will be the major factors
for the stainless steel foil substrates to be used for commercial applications. Promising candidates for the planarization
layer materials are reviewed and some of the properties are addressed. In addition, if the substrate is sustained to a
constant voltage for guaranteed circuit operation, capacitive coupling through the insulation and planarization dielectric
layer, from the conductive substrate to the electrode and circuit elements on it, is also carefully analyzed for panel
design and operation. Especially for large size high-resolution display applications, low k and thick planarization layer
should be used.
Our motivation is to realize CMOS on plastic foil. We report the development of thin film transistors (TFTs) made of nanocrystalline silicon (nc-Si:H). nc-Si:H is compatible with present a-Si:H thin film technology. Because of the structural evolution of nc-Si:H with film thickness, it requires extensive experimentation with device geometry. For comparison we fabricate TFTs in (a) conventional coplanar top-gate, top-source/drain geometry and (b) staggered top-gate, bottom source/drain geometry. A seed layer is introduced in the latter case serves to develop the crystallinity of the intrinsic channel layer. While the coplanar geometry provides the shortest carrier path in the most crystalline channel region, the inverted staggered geometry ensures that the active channel is formed in the last-to-grow nc-Si:H layer, and also avoids exposure of the channel to reactive ion etching (RIE). The highest process temperature is 150°C. Both intrinsic and doped nc-Si:H layers are grown by plasma-enhanced chemical vapor deposition with an excitation frequency of 80MHz. Present p-channel TFTs reach a hole field-effect mobility of ~ 0.2 cm2V-1s-1 in the staggered geometry, and an electron field-effect mobility of ~ 40 cm2V-1s-1 in both geometries. These results suggest that directly deposited nc-Si:H is an attractive candidate material for CMOS capable electronics on plastic substrates.