The use of backscatter electron detection in a wafer alignment system has been investigated. For certain types of wafer processing such an alignment system might show improved process robustness compared to optical sensors. This expectation is based on the principle that the alignment signal generated by backscattered electrons is formed by probing the volume of the alignment mark rather than its surface. This paper presents both simulations and experiment results on the viability and potential physical limitations of this alignment method. Physical properties of the backscatter electron alignment system are discussed. The results confirm that for some semiconductor wafer processes, this concept lives up to the expectations. The impact of electron beam energy, shot noise and alignment mark surface
roughness on process-induced alignment shift and aligned position
repeatability is investigated. In addition, the sensitivity to magnetic fields and mechanical vibrations has been investigated. The theoretically predicted relation between repeatability and illumination dose, due to shot noise has been experimentally confirmed. The results show that backscatter electron alignment is a promising alignment method, although a few issues remain unresolved.