NOVEL OBJECTIVE LENS FOR LOW-VOLTAGE ELECTRON-BEAM IMAGING

Low energy electron beams are increasingly being used in semiconductor manufacturing for wafer and mask inspection because of their low leve l of damage to the sample, and the reduced charging effects when the e lectron energy is close to E-II where secondary electron yield is unit y. Also due to the short range of the electrons with the sample and th e reduced proximity effect, electron beams with even lower energies ar e attractive for a variety of other applications such as surface studi es, thin film microscopy, and lithography. However, achieving high res olution and high secondary electron detection efficiency at 100 eV lan ding energy and below meets serious electron optical challenges. To ad dress 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 fo r minimum beam diameter and high secondary electron collection efficie ncy, subject to constraints imposed by practical concerns, such as arc ing. The lens was fabricated using a combination of conventional machi ning and silicon micromachining. By inserting the lens into the sample chamber of a traditional scanning electron microscope (SEM), we obtai ned images of many kinds of samples at landing energies as low as 20 e V. Resolution of better than 30 nm is achieved at 100 eV landing energ y. The degradation from the expected resolution (17 nm) is in part due to limitations imposed by the SEM resulting in a nonoptimal convergen ce angle. At these very low energies the image is seen to be very sens itive to contamination on the sample surface; this illustrates its pos sible application for viewing ultrathin films. We have calibrated the magnification versus landing energy relationship for the lens and carr ied out detailed signal and noise analyses for the electronics system. (C) 1997 American Vacuum Society.