USING A WEDGE OXIDE TO MONITOR LOW-ENERGY BEAM PURITY BY MEANS OF THERMA-WAVE MEASUREMENTS IN THE EATEN NV-8200P ION IMPLANTER

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., Nuc l. Instr. and Meth. B 74 (1993) 571]. This technique relies on the dif ferential hold up of the desired (low energy) and contaminant (high en ergy) ions in a screen oxide. In this study we have extended this tech nique by using a wedge oxide, varying from 0 to 1700 A 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 ox ide thickness required to completely trap the desired energy ions whil e at the same time allowing the maximum number of contaminant ions to pass through the oxide and produce a Therma-Wave signal in the underly ing silicon. Using an Eaten NV-8200P medium current implanter, we impl anted a 2 x 10(15) ions/cm(2) dose of boron at 3 and 10 keV and BF2 at 13.36 and 44.55 keV. We then compared these Therma-Wave values to the Therma-Wave values produced by reference ''contaminant'' boron implan ts, on other wafers, at 15 and 40 keV. These measurements indicate tha t 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.