Laser Crystallisation

By means of energy intensive laser irradiation, thin amorphous silicon layers (50-300 nm thickness) can be crystallized. The resulting polycrystalline silicon layers (grain sizes up to 3µm) can be used as seed layers for a subsequent epitaxial thickening of the silicon layer.

A Lambda Physik Xe-Cl Excimer Laser is in the center of the laser crystallization setup.

The most important technical properties of the the laser:

Laser energy flux density: bis zu 900 mJ/cm²
Wavelength: 308 nm
Frequency: 1-50 Hz
Pulse length: ~30 ns

The entire setup is simple. An optical homogenizer focuses the laser beam with an original spot size of 24x12 mm² onto a 6x6 mm² area. An optical attenuator is used to fine adjust the laser flux density. The sample is in a small reactor that can be evacuated (vacuum pressure < 10-5 mbar can be attained easily).

The sample head is mounted on an x-y-table such that the sample can be moved under laser beam in a defined way. The x-y-table, the laser and the attenuator are controlled by a computer. A camera is used for the positioning of the laser beam. The entire setup is installed on an optical table with active dampening of vibration.

Presently, the setup is used for the crystallization of thin amorphous silicon and silicon germanium layers (layer thickness 50-300 nm) as well as the pulsed laser deposition (laser ablation technique) of nominally intrinsic ZnO layers.

The thin polycrystalline layers serve as seed layers for a low temperature epitaxy (ECR-CVD) with which one can obtain sufficiently thick (a few µm) polycrystalline Si absorber layers.