Laser-assisted fabrication of bipolar transistors in silicon-on-sapphire

Abstract

The fabrication of bipolar junction transistors in silicon-on-sapphire (SOS) relies upon the laser-assisted dopant activation in SOS. A patterned 100% aluminum mask whose function is to reflect laser light from regions where melting of the silicon is undesirable is provided on an SOS wafer to be processed. The wafer is placed within a wafer carrier that is evacuated and backfilled with an inert atmosphere and that is provided with a window transparent to the wavelength of the laser beam to allow illumination of the masked wafer when the carrier is inserted into a laser processing system. A pulsed laser (typically an excimer laser) beam is appropriately shaped and homogenized and one or more pulses are directed onto the wafer. The laser beam pulse energy and pulse duration are set to obtain the optimal fluence impinging on the wafer in order to achieve the desired melt duration and corresponding junction depth. Care must be taken since activation and rapid dopant redistribution occurs when the laser fluence is above the melt threshold and below the ablation threshold. Thus, bipolar junction transistors in SOS utilize a pulsed laser activation of ion implanted dopant atoms. Appropriate masking and pulsed laser illumination assures the electrical activation of the dopant without allowing undesirable diffusion either vertically along crystallographic defects (diffusion pipes) or laterally.