Study On Focused Ion Beam Nano-fabrication Based On Molecular Dynamics Analysis And Microscopy Characterization

Focused ion beam(FIB)is a nanoscale ion beam operation approach,which integrates in situ material characterization,material removal and material deposition,and has a significant status in the feild of nanoscale fabrication.However,the formation of amorphous layers and Ga+ ions contamination layers during FIB milling would dramatically affects the optical,electrical,and mechanical properties of devices.To optimize processing technic of FIB,improve maching accuracy of FIB and ameliorate performance of devices,The formation of Ga+ FIB-induced damage and the process of subsequent annealing recovery were studied as well as surface modified induced by Ga+ ions implanted,and their micro mechanism were explored.Molecular dynamics(MD)simulation combined with nanoscale microstructure characterization was used to study the mechanisms of FIB nano fabrication and its annealing recovery on single crystal silicon,which is the typical brittle semiconductor material.A molecular dynamics model with the Tersoff–ZBL combined interatomic potential was built to investigate the dynamics of focused ion beam(FIB)milling and subsequent annealing in nanoscale time and space.The main research results are described as the following.To investigate the nanoscale damage during FIB milling with different ion energy(0.5 keV,1 keV and 2 keV)and different beam flow(1.1×1022 cm-2s-1~ 6.3×1025 cm-2s-1),system temperature and potential,radial distribution function,bond length distribution,bond angle distribution,and common neighbour analysis(CNA)were calculated and analyzed under various ion doses.The result of MD simulation indicated that both the depth and range of the defect region raised with the increase of ion doses and ion energies.The defect grew linearly at the beginning of ion implantation,when only point defects and dispersed amorphous pockets generated.Then the defects growth significantly slowed down before continued amorphous layer formed.The swelling of implantation region result from the combination of surface roughing and the decrease in the surface density.Compared with virginal smooth surface,rough surface after FIB milling can lead to an increase of height in atomic force microscopy(AFM)measurement.When ion doses were higher than 3.5×1023 ions/cm2,local temperature accumulation effect makes the temperature of silicon substrate surface higher than the melting point and the sputtering yield increase.Monte Carlo method was applied to study the effects of ion implantation incident angle and the beam energy on the sputtering yield,distribution of ions and the target material damage.These research results make up the deficiency of single crystal silicon Ga-FIB nano machining mechanism.As the surface microstructure and surface microchemistry was observed and detected by XPS,Raman,electron microscopy and AFM,the influence of focused ion beam(FIB)processing parameters on the irradiation damage layer was revealed.From the XPS Ga depth profiles and TEM results,the etch resistance of the FIBI layer with high ion doses was attribute to the continuous nanoscale Ga clusters on the amorphous layer.This discovery solved the key issues about FIB XeF2 gas-assisted etching(FIB-GAE)mask in the vacuum under the anaerobic environment.Annealing recovery of nanoscale silicon surface damage caused by Ga focused ion beam was investigaged by MD simulation.MD results show that the recrystallization lead to a c-Si regrowth processes both from the bottom towards the top surface and from periphery to centre.The profiles of CNA damage area represented a shortest recovery time of 2.0 ns at the temperature of 0.863?Tmc.The recrystallization velocity was increased with annealing temperature far below the melting(<0.863?Tmc).Both melting on the top surface and recrystallization at crystalline/amorphous interface have existed as annealing at 0.94?Tmc,which is near the melting point.Ga ions migrated together and were swept by the c-Si/a-Si interface during annealing.The content of precipitated Ga increased with temperature.The precipitation of Ga ions left sunken at the surface after annealed.Post annealing experiments after Ga-FIB milled on c-Si were conducted in Vacuum Tubular Furnace.The influence of annealing temperature and holding time on surface recovery was evaluated.The influence of the migration and precipitation of Ga on recrystallization and surface topology during annealing was revealed.The experiment shows that these implanted Ga+ ions could migrate to the surface of the implanted areas during annealing and completely segregate from the Si after annealing at 800? for 30 min.Annealing process also contribute to a recrystallization of amorphous silicon to polycrystalline silicon.Besides,when the ion doses is as high as 1×1018ions?cm-2,polycrystalline silicon is observed at the surface even after low temperature(300?)annealing due to the precipitation of Ga.