PLASMA ION-IMPLANTATION OF NITROGEN INTO SILICON - CHARACTERIZATION OF THE DEPTH PROFILES OF IMPLANTED IONS

The in-depth concentration distribution or depth profile of nitrogen implanted into silicon wafer substrates using plasma ion implantation (PII) is studied using secondary-ion-mass spectrometry and Auger electron spectroscopy sputtered depth profiling. Plasma ion implants were performed using a low-pressure (5×10-5 Torr) collisionless plasma at voltages of 50 and 100 kV to a fluence of 1.5×1017 cm-2 using voltage pulses 10 μs in duration, with 1 μs rise time, and at a repetition rate of 200 Hz. The measured depth profiles are compared with those from both conventional ion-beam implantation and numerical simulations. The comparisons indicate an incident flux composed of ∼90% N+2 and ∼10% N+ ions. Compared with ion-beam implants, which exhibit a nearly Gaussian-shaped depth profile, the plasma ion implantation profiles are"filled in" with an approximately constant nitrogen concentration for depths less than the predicted ion range. The profiles are modeled assuming that incident ions have a distribution of energies. Thermal diffusion does not seem to affect the nitrogen profile. Energy distributions are determined using a series of simulated implants at different energies as basis functions to numerically fit the measured profiles. The normalized energy distribution is independent of voltage at 50 and 100 kV, increases monotonically with increasing energy, and indicates that ∼50% of the ions are implanted at energies <80% of the nominal value. Comparisons with several developed models of the PII process suggest that displacement current effects within the expanding ion sheath may significantly affect the incident ion energy distribution.