Demonstration of Secondary Electron Detection using Monolithic Multi-Channel Electron Detector

Abstract

The throughput has long been and still is a major issue in electron-beam systems such as those used for lithography and inspection. We have previously described a method which generates multiple primary beamlets and focuses them simultaneously by applying a uniform axial magnetic field. This method allows a significant increase in total current without the growth of Coulomb blurring, by virtue of the absence of a crossover [1,2].As with other multiple electron beam systems, secondary electron detection is one of the key challenges for this method. In order to meet this challenge, we have developed a multi-channel PIN-diodebased detector (Fig. 1). The electrodes, in a linear array with pitch of 250 ptm, detect the signals generated by the secondary electrons incident into the other side of the detector. Each detection area has a throughhole, which allows the primary beamlet to pass through. We have already demonstrated the capability of parallel detection of secondary electrons stimulated by multiple primary beamlets, as well as a modest internal amplification and fast response time [3]. These characterizations were performed by using a primary beamlet of a conventional SEM instead of secondary electrons.The secondary electron detection scheme using this detector is shown in Fig. 2. The secondary electrons generated at the sample surface are confined to lateral dimensions by a uniform axial magnetic field which immerses the whole system. A bias, which is applied between the detector and the field terminator, lowers the landing energy of the primary beamlets and increases the detector gain by accelerating the secondary electrons. The field terminator, which is equipotential with the sample, suppresses the vertical field, i.e., accelerating field for secondary electrons. The deflector, which generates a static transverse field between the field terminator and sample, deflects the secondary electrons before being accelerated so that they hit the active area of the detector array. When the shape and layout of the throughholes are arranged appropriately, the secondary electron detection rate can be as high as 95 % and the aberrations of the primary beamlets can be kept as small as 10 nm [4].As a feasibility check, the secondary electron detection has been demonstrated and the confinement of the secondary electrons has been observed with an arrangement consisting of a sample, a quadrupole deflector, field terminator and a detector array ( Fig. 3(a)). The sample and the field terminator are negatively biased relative to the detector array. One of the four electrodes of the deflector can be positively biased with respect to the other three, which are connected to the sample. This arrangement is different from our ideal configuration. First, a uniform axial magnetic field is not applied; therefore, the secondary electrons are less confined than they would be with a magnetic field of the order of 0.