基于原子层沉积的氧化铝和氧化镁纳米薄膜二次电子发射特性

Secondary electron emission of Al_(2)O_(3) and MgO nanofilms fabricated by atomic layer deposition

氧化铝、氧化镁具有高二次电子产额(secondary electron yield,SEY)特性,能够作为电子倍增器中打拿极的镀层材料以大幅提升器件的电子倍增效率.本文使用原子层沉积工艺在低阻硅表面分别制备了7组氧化铝、氧化镁纳米薄膜(1~50 nm),并在20 nm氧化镁薄膜表面制备了5组氧化铝纳米薄膜(1~20 nm);表征了薄膜的成分、形貌、厚度和SEY特性,分析了薄膜厚度对样品SEY的影响规律.结果表明,对于硅基底氧化铝和氧化镁纳米薄膜,其SEY随薄膜厚度增加而逐渐增大,但增量逐渐降低;当膜厚超过30 nm时,SEY增量趋于0,说明此时入射电子的侵入深度已经小于膜厚.双层结构SEY半物理理论计算结果表明,纳米薄膜厚度对SEY有显著影响,入射电子能量越低、薄膜越厚,SEY受膜厚影响越明显;随入射电子能量增大,薄膜厚度对SEY的影响逐渐变小,这是由于电子的侵入深度也变大.本研究通过实验结合理论深入分析了纳米级超薄氧化铝和氧化镁薄膜的SEY特性,对于开展电子倍增器中高SEY纳米介质薄膜应用研究具有重要意义.

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.Al_(2)O_(3) 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 Al_(2)O_(3) 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 Al_(2)O_(3) 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 Al_(2)O_(3) /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 Al_(2)O_(3) 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.