Beam Angular Divergence Effects in Ion Implantation

Abstract

An important difference between monomer ion beams and heavy molecular beams is a significant reduction in beam angular divergence and increased on‐wafer angular accuracy for molecular beams. This advantage in beam quality stems from a reduction in space‐charge effects within the beam. Such improved angular accuracy has been shown to have a significant impact on the quality and yield of transistor devices [1,12]. In this study, B18Hx+ beam current and angular divergence data collected on a hybrid scanned beam line that magnetically scans the beam across the wafer is presented. Angular divergence is kept below 0.5 deg from an effective boron energy of 200 eV to 3000 eV. Under these conditions, the beam current is shown analytically to be limited by space charge below about 1 keV, but by the matching of the beam emittance to the acceptance of the beam line above 1 keV. In addition, results of a beam transport model which includes variable space charge compensation are presented, in which a drift mode B18Hx+ beam is compared to an otherwise identical boron beam after deceleration. Deceleration is shown to introduce significant space‐charge blow up resulting in a large on‐wafer angular divergence. The divergence effects introduced by wafer charging are also discussed.