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Shallow p-n junctions produced by laser doping with Boron and Phosphorus Silicate glass

: Bollmann, D.; Stock, G.; Neumayer, G.; Haberger, K.

Laser '91. Laserion, laser in science. International Laser Symposium
International Laser Symposium <1991, München>
Fraunhofer IFT; 2000 dem IZM eingegliedert
diffusion; doping; Dotierung; flach; flat; Halbleiter; laser; Oberfläche; p-n junction; p-n Übergang; semiconductor; shallow; silicon; Silizium; surface

For further reduction of IC dimensions, the lateral as well as the vertical doping profiles have to be controlled with an accuracy of better than 0.05 mym. These shallow profiles can hardly be achieved by ion implantation or classical diffusion. Laser driven outdiffusion from solid source layers could be a promising technique for the production of very flat p-n junctions. We used a high power CO2 laser with a wavelength of 10.6 mym. It was pulsed with an energy of 3 J and a pulse duration of 50 ms. A cylindrical lense made a focus with rectangular shape (10 mm x 1mm) with a power density of 5 x 10 high 3 W/qcm. The source layer was boron or phosphorus silicate glass (BSG or PSG) produced by chemical vapour deposition. The B and P concentration in the source layer was about 4%, which corresponds to 2 x 10 high 21 cm high -3. It was in contact to the substrate only in the windows of a masking oxide layer (Fig. 1). The wafer was moved by an X-Y-table which results in a scanning, of the la ser beam over the surface. By variing the scan speed by constant pulse rate the overlapping was changed from a factor of 1 to 30. The substrate was preheated to 450 degree C in order to reduce thermal stress. The dopant profiles were determined by SIMS, spreading resistance and RBS. The measured maximal concentrations in the doped layer were in the range of 5 x 10 high 19 cm high -3. The resulting depth of the p-n junction was about 0.1 mym, (Fig. 2). The dependence of the depth versus number of pulses was not so strong as expected from theoretical considerations. This suggests a saturation effect of the short time diffusion. Parallel to the experiments the resulting dopant distribution was simulated by the program ICECREM. Also the transient temperature distribution in the silicon wafer was simulated.