Novel objective lens for low voltage electron beam imaging

Abstract

Low energy electron beams are increasingly being used in semiconductor manufacturing for wafer and mask inspection because of their low level of damage to the sample, and the reduced charging effects when the electron energy is close to EII where secondary electron yield is unity. Also due to the short range of the electrons with the sample and the reduced proximity effect, electron beams with even lower energies are attractive for a variety of other applications such as surface studies, thin film microscopy, and lithography. However, achieving high resolution and high secondary electron detection efficiency at 100 eV landing energy and below meets serious electron optical challenges. To address this issue, we describe a low aberration objective lens that is combined with an efficient secondary electron detector. The objective lens has a final electrode just in front of the sample to minimize the electric field at the sample surface. We have optimized the design for minimum beam diameter and high secondary electron collection efficiency, subject to constraints imposed by practical concerns, such as arcing. The lens was fabricated using a combination of conventional machining and silicon micromachining. By inserting the lens into the sample chamber of a traditional scanning electron microscope (SEM), we obtained images of many kinds of samples at landing energies as low as 20 eV. Resolution of better than 30 nm is achieved at 100 eV landing energy. The degradation from the expected resolution (17 nm) is in part due to limitations imposed by the SEM resulting in a nonoptimal convergence angle. At these very low energies the image is seen to be very sensitive to contamination on the sample surface; this illustrates its possible application for viewing ultrathin films. We have calibrated the magnification versus landing energy relationship for the lens and carried out detailed signal and noise analyses for the electronics system.