Patent ID: 6870170

Claim:
An ion implanter comprising: a vacuum chamber; a substrate holder in the vacuum chamber operative to support a target substrate; an ion beam generator operative to generate a beam of ions including a species to be implanted in the target substrate; a scanning apparatus operative to effect a relative scanning movement between the target substrate and the ion beam; a vacuum pump operative during implantation to pump residual gas from the vacuum chamber; the scanning apparatus being arranged to produce during an implant repeated scans of the beam over the target substrate separated by periods when the beam does not impinge on the target substrate, whereby residual gas pressure in said chamber rises during each of said scans due to outgassing from the target substrate and falls exponentially with a pump down time constant towards a minimum pressure value during said separating periods due to pumping by said vacuum pump, at least some of said separating periods having insufficient duration relative to said pump down constant to allow the residual gas pressure to reach said minimum pressure value; an ion current detector operative to provide multiple time spaced measurements of the ionic current in the ion beam at the target substrate during each of a plurality of said separating periods including said periods having insufficient duration; a controller operative to control said scanning apparatus to produce said repeated scans and being operative to provide at least one extended said separating period which has sufficient duration to allow the residual gas pressure to be pumped down to said minimum pressure value, said ion current detector providing said multiple time spaced measurements during said extended separating period; and a processor operative (a) to receive said multiple measurements from each of said separating periods including said extended separating period, (b) to identify from said ionic current measurements during said extended separating period, a maximum measured beam current value corresponding to the residual gas pressure reaching said minimum pressure value, (c) to calculate from the multiple time spaced measurements received during said extended separating period and said identified maximum measured beam current value, an estimate of said pump down time constant, and (d) to calculate from said received measurements during each of said plurality of separating periods subsequent to said extended operating period and said estimate of said pump down time constant, repetitive corrected values for the total beam current representing the rate of delivery in the beam of particles of the species to be implanted.