Laser crystallization or crystal structure alteration of amorphous or polycrystalline semiconductor layers comprises paired laser pulse irradiation for extended melt time while maintaining a low substrate temperature

摘要

Amorphous semiconductor layer crystallization or polycrystalline semiconductor layer crystal structure alteration is achieved by paired laser pulse irradiation to provide an extended melt time while maintaining a low substrate temperature. Crystallization of amorphous semiconductor layers or alteration of the crystal structure of polycrystalline semiconductor layers is carried out by irradiation with laser pulses (LP1, LP2) from two pulsed laser beam sources. The laser pulse pair has the following properties: (a) the pulses overlap or pulse LP2 follows pulse LP1 by a short spacing (shorter than the melting time of the target material); (b) pulse LP1 (preferably with a typical width of 10-50 nsec.) causes brief total or partial melting of the irradiated area, while pulse LP2 is homogeneous and is absorbed in the target surface region melted by LP1 to prolong the melting time; (c) pulse LP1 has a wavelength such that it reaches a depth less than or equal to the layer thickness, while pulse LP2 has a wavelength of LP2 such that it reaches a depth in the micron range in the solid material but only a few (up to 30) nm in the liquid material so that the liquid material selectively absorbs the radiation; (d) the intensity, pulse width and pulse spacing have such values that, at similar fluences (F = intensity x pulse width, i.e. I * tau ) of LP1 and LP2, the intensity of LP2 is large enough to avoid re-solidification of the melt after the end of LP1, with the proviso that tau 1 is less than tau 2, I1 is greater than I2 and the ablation threshold for LP2 restricts its fluence.