LASER-INDUCED FORMATION OF POROUS SILICON

The mechanism of photoelectrochemical porous silicon formation in fluoride solutions under laser illumination and dark conditions has been investigated. Experiments were performed in a PTFE cell with a plastic window to allow the laser path to be horizontal and the silicon electrode to be in a vertical position inside the cell. Dark and illuminated anodic and cathodic current-voltage (I-V) curves were both measured in real time by chopping the laser beam. N-type wafers of resistivities 0.001 to 22 Omega . cm have been investigated for various conditions of illumination intensity and polarization. We focused our attention on relatively low illumination intensities similar to 10(-5)-10(-8) W/mm(2) and high HF concentration. By measuring the dissolution rate and the photogenerated current we estimated that the main reaction path requires two holes for each Si atom as: Si + 2HF + 2h(+) --> SiF2 + 2H(+) hv. The utilization of this technique for direct projection printing of porous Si images of 10 Crm resolution was demonstrated.