Excimer laser annealing of microcrystalline silicon

AbstractThe development of flexible displays and screens is now a major target of the industry. This purpose requires the elaboration of pliant substrate compatible processes. The main obstacle is the vulnerability of such substrates toward heat. Most parts of usual processes are able to be performed at low temperature, but polycrystalline silicon deposition always involves high temperature. Polycrystalline silicon is necessary to reach high mobility, which, in the case of flexible screens, is critical to decrease the area of the pixel circuit and then to reach high aperture ratio. One solution consists in low temperature deposition of microcrystalline silicon followed by an excimer laser crystallization phase. Microcrystalline silicon films were deposited at low temperature (165 °C maximum) by radio‐frequency plasma enhanced chemical vapour deposition on glass substrate covered with silicon dioxide film and on polyethylene naphtalate covered with silicon dioxide film as flexible substrate. These microcrystalline silicon films were excimer laser crystallized, using a step‐by‐step method. Polyethylene naphtalate substrates underwent crystallization without showing any damage. Using deposits made on glass, the crystallization technique has been optimised to obtain the highest crystalline quality starting from 200 nm thick microcrystalline silicon films. On each kind of substrate, crystallized films were characterized using Raman spectroscopy, scanning electron microscopy and atomic force microscopy. Raman spectrum shows very sharp transverse optic peak at 521 cm–1. The small full width at half maximum, 4 cm–1, is close to the value for single crystalline silicon. Scanning electron microscopy and atomic force microscopy reveal silicon grains with size over 400 nm on glass and 100 nm on polyethylene naphtalate. Thin film transistors with a mobility of 400 cm²/V.s on glass and 46 cm²/V.s on polyethylene naphtalate have been produced from such crystallized layers. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)