Simulation of backscattered electron signals for x-ray mask inspection

The successful use of an electron-beam lithography system for the inspection of x-ray masks requires an in depth understanding of the scattering properties of the defects. A new Monte Carlo computer program has been developed to simulate the backscattered electron signal from various types of two-dimensional gold-on-silicon structures. Simulation results for a variety of submicron patterns show excellent agreement with experimental backscattered electron signals from an annular silicon diode detector. It is found that the simulated energy signal, rather than the number signal, gives the best results. The program is used to optimize the placement of the annular detector by examining the take-off angle distributions from different structures. The optimum take-off angle range for this detector is found to be 30°–57.5° as opposed to the lower angle range favored for registration signals. It is also found through this investigation that small steps of gold on silicon will be the hardest defects to detect for this optimum take-off range.