Secondary electron emission of Al2O3 and MgO nanofilms fabricated by atomic layer deposition

Electron multiplier devices are widely applied in many electronic instruments like mass spectrometers and atomic clocks. It is considerably crucial for a multiplier to possess a high electron gain, and this index can be directly determined by secondary electron yield (SEY) of the dynodes. Al2O3 and MgO possess a relatively high SEY level among majority of dynode materials, and their film products are excellent dynode candidates. Whereas, for some multipliers like microchannel plate (MCP), only an ultrathin film of several nanometers is allowed to be coated onto the inner wall of the micro channels to avoid the variation of the channel diameter. Therefore, SEY characteristics of the ultrathin films are necessary to be figured out. Here. by using the technology of atomic layer deposition 7 groups of ultrathin Al2O3 and MgO nanofilms with increase thickness (1, 3. 5, 7, 10, 30, and 50 nm) are fabricated on silicon (Si) substrates. As well as, 5 groups of Al2O3 nanofilms (1, 2, 3, 4, and 20 nm) are deposited on MgO film (20 nm) substrate. Surface composition, morphology, film thickness, and SEY have been characterized in detail. Via the experiments, it is found that SEY of the Al2O3/Si and MgO/Si samples largely depends on the film thickness, namely. SEY increases obviously as the film thickness rises, meanwhile. the increment of SEY decreases gradually. The SEY tendency indicates that the effect of top film on SEY becomes enhanced. and the influence of bottom substrate on SEY becomes weakened. When the film thickness increases beyond 30 nm. SEY increment approaches to 0, and SEY tends to be saturated. This phenomenon demonstrates that the penetration depth of incident electrons is less than the film thickness under the circumstances. To interpret the experimental results, the SEE semi-physical theory developed for double layer structures is utilized. The calculation results indicate that the film thickness has a remarkable impact on SEY, especially when the incident energy becomes lower and the film becomes thicker. the results also reveal that the dielectric surface film possesses a great ability to modulate the surface SEY. However, SEY becomes less dependent on film thickness as the incident energy increases. and it results from the increase of penetration depth for the incident electrons. This work reveals the mechanism of the SEE characteristics for ultrathin Al2O3 and MgO nanofilms. which is of great significance for the subsequent research on the use of nanoscale high SEY dielectric films as the SEE functional layer in electron multipliers.