A thermal model for beam-induced melting of polysilicon in polysilicon/insulator/silicon structures has been used to compare CW Ar+ laser and electron-beam melting processes. The laser melting process exhibits a decreasing recrystallization threshold power and an increasing substrate melting threshold power with increasing insulator thickness. The decrease in recrystallization threshold with increasing insulator thickness is generally smaller for electron beams because of their deep penetration. The substrate melting threshold is approximately independent of insulator thickness for e-beams since no change in the power absorption occurs on melting of the polysilicon. As a result, the process window between the onset of recrystallization and melting of the substrate is narrower for e-beams than for laser beams. Also, essentially no window will exist between the onset of melting over a seed and substrate melting under an oxide if lateral epitaxy is performed with an electron beam.