The significance of controlling “off-axis” (from 1-0-0) oriented Si wafers during high angle implants

Various “off-axis” (from 1-0-0) angle oriented (biased) silicon wafers were implanted at high tilt angles (up to 600) to investigate the effects on junction depth and sheet resistance repeatability. “Off-axis” orientations of 0.0°, 0.35°, 1.0°, 4.0°, and 7.0° were implanted with As⁺ , B⁺, or P⁺ at a dose of 5e13 ions/cm² with an energy of 40 keV, 9 keV, or 23 keV respectively The tilt angles explored were 0°, 30°, 45°, and 60° with the implanter “set” dose and energy geometrically corrected for (by 1/cos θ). This geometrical correction for dose and energy should provide the same delivered substrate dopant dose and expected ion projected range regardless of tilt angle, thus isolating the effects resulting from other variables including the “off-axis” wafer cuts. Results indicate that as the tilt angle increases the need for tight control of “off-axis” sliced silicon substrates is less important in producing repeatable junctions as long as the tilt angle reproducibility is good. However, channeling, reflection loss and post-implant process variations play a significant role in junction depth and sheet resistance repeatability. Based on these results it is recommended that all process variables (e.g. energy, dose and anneal) at high tilt angle be empirically determined and stringent controls employed to yield reproducible junction depth and sheet resistance values. With all of these controls in place reproducibility was best at the higher tilt angle of 60° (0.65%) because of reduced channeling variations