1.School of Automation & Information Engineering, Xi’an University of Technology, Xi’an 710048, China 2.Department of Electronic Science and Technology, Xi’an Jiaotong University, Xi’an 710049, China
Abstract:The electron beam induced current (EBIC) characteristics of dielectric/semiconductor thin films under the electron beam (e-beam) irradiation is the important means of implementing the electron microscopic detection. The transient EBIC characteristics of the SiO2/Si thin film irradiated by a high-energy e-beam are investigated by combining the numerical simulation and the experimental measurement. The scattering process of electrons is simulated by the Rutherford scattering model and the fast secondary electron model, and the charge transport, trapping and the recombination process are calculated by the current continuity equation and the Poisson equation. The transient charge distribution, EBIC and the transmission current are obtained, and influence of the beam current and the beam energy on them are analyzed. The results show that due to the electron scattering effect, the free electron density decreases gradually along the incident direction. The net charge density near the surface is positive and negative along the incident direction because of secondary electrons (SEs) emitted from the surface, and therefore the electric field intensity is positive near the surface and negative inside sample, which causes some electrons to be transported to the substrate and some SEs return to the surface. The negative charge density at the SiO2/Si interface is higher than that in the nearby region because some electrons are trapped by the interface trap. With the decrease of the net charge density with e-beam irradiation, the charging intensity decreases gradually. Meanwhile, electrons are gradually transported to the substrate, and consequently EBIC and the sample current increase and the electric field intensity decreases with e-beam irradiation. However, due to the weak charging intensity, the surface emission current and the transmission current remain almost invariant with e-beam irradiation. The EBIC, the transmission current and the surface emission current are approximately proportional to the beam current. For the SiO2/Si thin film in this work, the transmission current increases gradually to the beam current value with the increase of the beam energy, and the EBIC presents a maximum value at the beam energy of about 15 keV. Keywords:numerical simulation/ electron beam induced current/ trapping/ transport
由于束流对电子散射、输运特性的影响较小, 因而平衡态时电子束感生电流IEBIC与束流呈现近似正比例关系, 如图9所示. 类似地, 表面出射电子电流、样品电流随束流变化也呈现相同的正比例关系. 图 9 模拟得到的不同束流时电子束感生电流IEBIC的稳态结果 Figure9. Simulated IEBIC in the steady state under the different values of beam current IB.
束能不仅决定电子产额也决定电子入射深度, 对带电特性的影响较为复杂. 图10是模拟得到的不同束能下透射电流ITE时变特性, 当束能从10 keV增大到20 keV时, 由于电子入射深度的增大, 透射电子电流逐渐增大; 但从20 keV增加到30 keV时, 透射电子电流略小于入射电子电流IB的稳定值. 这是因为, 在较高入射能量段, 背散射电子产额较低, 绝大多数入射电子将直接穿透样品, 因而透射电流接近于束流(1.6 nA). 图 10 模拟得到的不同束能时透射电流ITE的时变特性 Figure10. Simulated ITE as a function of the irradiation time in the different beam energies EB.
图11是模拟得到的不同束能时电子束感生电流IEBIC的时变特性, 在约15 keV处呈现极大值. 这里, 当束能从10 keV升至15 keV, 经非弹性散射激发的次级电子数量较大, 导致EBIC增大. 而随着束能的继续升高, 尽管次级电子数量较大但样品内激发的次级电子分布更为分散, 内部电场逐渐减小, 导致电子向下输运能力减弱, 因而EBIC反而减小, 故而在束能约为15 keV时EBIC呈现极大值. 图 11 模拟得到的不同束能时IEBIC的时变特性 Figure11. Simulated IEBIC as a function of the irradiation time in the different beam energies EB.