Oxide-silicon-oxide buffer structure for ultralow temperature polycrystalline silicon thin-film transistor on plastic substrate

A novel oxide-silicon-oxide buffer structure to prevent damage to a plastic substrate in an ultralow temperature (<120/spl deg/C) polycrystalline silicon thin-film transistor (ULTPS TFT) process is presented. Specifically, an amorphous silicon film was inserted as an absorption layer into buffer oxide films. The maximum endurable laser energy was increased from 200 to 800 mJ/cm/sup 2/. The fabricated ULTPS nMOS TFT showed a performance with mobility of 30 cm/sup 2//Vs.

Flexible displays are currently the subject of a great deal of interest due to their low weight and superior compactness, robustness, and design flexibility compared to conventional glass-based displays. In order to realize a mechanically stable display, however, several issues have to be resolved [1]. Inorganic materials have very different mechanical properties compared to plastic substrates, and therefore cracking and delamination may occur during the fabrication process and in operation. Also, the thermal expansion coefficients of plastic substrates are much larger than those of inorganic materials, and stresses during the deposition process are consequently introduced. If a freestanding substrate is employed, the curvature of inorganic film structures on plastic substrates is changed during circuit fabrication, causing misalignment between layers. In order to prevent misalignment, the plastic substrate is typically bonded to a rigid carrier of a glass substrate. The adhesive used for bonding must withstand the thin-film transistor (TFT) process temperatures and chemicals. As such, the use of an adhesive necessitates a reduction of the maximum process temperature from the value that the substrate itself would withstand. The aforementioned issues as well as the low thermal resistance of most plastic substrates necessitate an ultralow temperature polycrystalline silicon (ULTPS) TFT process [2]–[7]. The device characteristics fabricated on a plastic substrate show mobility values in the range of 15–250 for nMOS TFTs [3], [5], [7] and 25–66 for pMOS TFTs [4], [6].