Ion beam blanking apparatus and method

In an ion beam apparatus including

• (A) a beam generating means for generating a beam of ions to propagate substantially toward a workpiece during unblanked periods of operation and
• (B) a beam blanking means for blanking the beam during selected blanked periods of operation, wherein the beam blanking means includes (i) an aperture means, including an aperture element, for providing an aperture through which the beam passes to propagate to the workpiece during unblanked periods of operation, and (ii) a beam deflecting means for deflecting the beam during the blanked periods so that the beam is deflected away form the aperture and strikes the aperture element, whereby the beam is prevented from propagating to the workpiece, the improvement comprising
• potential barrier means for generating an electric potential barrier between the aperture means and the beam deflecting means, said potential barrier including at least one barrier electrode having a selected negative potential applied thereto, exceeding the kinetic energy of secondary charged particles emitted form the aperture element due to ions striking the aperture element during the blanked periods, so that
• (i) secondary charged particles emitted from the aperture element during the blanked periods are prevented from striking the beam deflecting means,
• thereby reducing any of hydrocarbon cracking, insulator accumulation, and charge accumulation on said deflecting means due to impacts of the secondary charged particles on the beam deflecting means, and enhancing ion beam positional accuracy during unblanked periods of operation, and
• (ii) the charged particles emitted from the aperture element are returned to the aperture element,
• and also comprising
• first current measuring means, electrically coupled to the aperture element, for generating a first electrical signal representative of the current of ions flowing to the aperture element during blanking, minus the current of secondary ions desorbed from said aperture element,
• second current measuring means, electrically coupled to said at least one barrier electrode, for generating a second electrical signal representative of the current of secondary ions desorbed from said aperture element and striking said at least one barrier electrode, and
• summing means, electrically connected to said first current measuring means and said second current measuring means, for generating an electrical signal representative of the sum of said first and second electrical signals, said sum being representative of ion beam current.