Large-area shower implanter for thin-film transistors

Solid-state diffusion and conventional ion implantation are not suitable for source and drain regions formation of polysilicon thin-film transistors on glass substrates. A 30 cm diameter large-area low-energy ion shower implanter with RIPE ion source and double-grid extraction system was developed as a possible low-cost solution. The ion beam current density for hydrogen plasma was 100 μA/cm² for 3 keV implant energy, 300 W RF power, 140 gauss magnetic field and 3×10^-4 mbar pressure. The uniformity of beam current density over the central 20 cm diameter was ±3.5%. Phosphorus implantation has been performed using a 15% PH₃ in H₂ gas mixture. Implantation at 3 keV for 5 min. results in an integrated dose of 2.48×10¹⁶ cm^-2 with the concentration peak at a depth of 8.3 nm. Planar and mesa diodes fabricated on p-type silicon substrates have yielded fine rectifier characteristics. The shower implanter is thus suitable for TFTs source and drain region formation