Inadequate pattern-placement accuracy is becoming a limiting factor for conventional scanning-electron-beam lithography (SEBL) in many applications, such as mask writing and fabrication of integrated optical devices. Spatial-phase-locked electron-beam lithography (SPLEBL) with continuous feedback promises sub-1-nm placement accuracy by monitoring and correcting the beam position in real-time during exposure [J. I Hastings, F. Zhang, and H. I. Smith, J. Vac. Sci. Technol. B 21, 2650 (2003)]. Ideally, SPLEBL provides a continuous reference signal to infer and correct beam position. Hence, the beam current must be reduced by a factor of 10 or more when traversing areas that are not to be exposed. To accomplish this we have investigated a partial-beam-blanker which consists of a quadrupole lens and a current-limiting aperture. This design has several desirable features. First, it does not shift the center of mass of the beam, so that spurious deflection can be avoided. Second, the operating voltage of the device is very low, which makes it suitable for high modulation rate. Third, this device can be easily incorporated into existing SEBL system design. In this article, the theoretical background will be provided, and the effect of the quadrupole lens on the beam probe size evaluated. (C) 2005 American Vacuum Society.