An electron beam device for inspecting a target substrate or specimen thereon includes a beam separator with an asymmetric quadrupole electrostatic deflector for improving field uniformity for a single direction of deflection. The asymmetric quadrupole electrostatic deflector includes two orthogonal electrode plates spanning roughly 60 degrees and two electrode plates spanning roughly 120 degrees, the two latter plates defining a unidirectional deflection field. The device generates a primary electron beam and focuses the primary electron beam along an optical axis into the target substrate. Secondary electrons detected at the target substrate are focused into a secondary electron beam. The beam separator with asymmetric quadrupole electrostatic deflector deflects the secondary electron beam away from the axis of the primary electron beam in the direction of deflection and into a detector array.