Using a wedge oxide to monitor low energy beam purity by means of therma-wave measurements in the Eaton NV-8200P ion implanter

Abstract

A technique using a Therma-Wave Therma-Probe has been developed which allows the possibility of the detection of parts per million levels of high energy contaminants in low energy beams [C. Jaussaud et al., Nucl. Instr. and Meth. B 74 (1993) 571]. This technique relies on the differential hold up of the desired (low energy) and contaminant (high energy) ions in a screen oxide. In this study we have extended this technique by using a wedge oxide, varying from 0 to 1700 Å as the hold-up oxide. This approach, instead of using a single oxide thickness across the wafer, makes possible the empirical determination of the exact oxide thickness required to completely trap the desired energy ions while at the same time allowing the maximum number of contaminant ions to pass through the oxide and produce a Therma-Wave signal in the underlying silicon. Using an Eaton NV-8200P medium current implanter, we implanted a 2 × 10 15 ions/cm 2 dose of boron at 3 and 10 keV and BF 2 at 13.36 and 44.55 keV. We then compared these Therma-Wave values to the Therma-Wave values produced by reference “contaminant” boron implants, on other wafers, at 15 and 40 keV. These measurements indicate that the wedge oxide allows for the determination of the energetic purity of the beam at less than or equal to the 5–10 ppm level.