Beam parallelism in the 8250 medium current implanter

Small variations in beam angle across the wafer can affect the apparent dose uniformity and implant depth under conditions where channeling of the implanted ions is possible. The depth profile of the implanted species can vary significantly under certain crystal orientations even with beam angle variations as small as ±0.5 degrees. This leads to non-uniformity in the measurements made with thermal wave or sheet resistance probes. The 8250 uses a second-generation design of the electrostatic scanning and parallelizing lens to produce a horizontally scanned beam in which the average beam angle at any point on the wafer is typically the same within 0.2 degrees. This paper describes the method of measurement of the beam angles and shows the effect of small angle variation on implanted wafers