﻿ AFM变载荷刻划硅基底的分子动力学研究
 光学仪器  2020, Vol. 42 Issue (2): 57-63 PDF
AFM变载荷刻划硅基底的分子动力学研究

Molecular dynamics simulation of AFM scratching on silicon with varying load
MA Yan, PENG Jun
School of Physics Science and Engineering, Tongji University, Shanghai 200092, China
Abstract: A series of molecular dynamics (MD) simulation are performed to study the surface deformation behavior of silicon substrate scratched by an atomic force microscopy (AFM) probe. A modified MD model is established, a quantitative index is proposed to describe the pile distribution, and the structure recognition algorithm is used to reveal the generation process of non-crystal layer. On these bases, the effects of scratching velocity, tip radius and probe wedge angle on the scratching process are investigated. Results show that (1) The scratching velocity has little effect on the groove surface. The piles on substrate surface are the least when the scratching velocity is less than 0. 3 nm/ps or greater than or equal to 1.5 nm/ps. (2) The probe wears and tears when the tip radius is less than or equal to 1 nm. The probe deforms elastically when the tip radius is greater than or equal to 1.5 nm. The tip radius should be 2-3.5 nm for the best scratching results. (3) The large wedge angle helps to reduce the piles distributed on the substrate surface.
Key words: AFM    varying load    scratching    silicon    molecular dynamics

1 模拟方法 1.1 模型的建立

1） 区域四周采用周期性边界条件，以消除边界效应和尺度造成的影响；

2） 探针按照实际的形状设为四面锥形，并设为非刚体，使之能够描述探针本身的形变，如针尖的磨损；

3） AFM在接触模式下有一个预置的z偏置，以使针尖靠近基底表面，故探针整体有个18°的倾斜。

 图 1 AFM刻划模型 Figure 1 The model of AFM scratching

 图 2 探针模型 Figure 2 The model of AFM probe

1.2 势函数的选择

 $E = {D_0}[{{\rm e}^{ - 2\alpha (r - {r_0})}} - 2{{\rm e}^{ - \alpha (r - {r_0})}}]$ (1)

Tersoff势的势函数形式为

 $U = \sum\limits_{i < j} {\sum {{f_c}({r_{ij}})[{a_{ij}}\exp ( - {\lambda _{ij}}{r_{ij}}) - {b_{ij}}\exp ( - {\mu _{ij}}{r_{ij}})]} }$ (2)

Tersoff势的参数来自文献[16]。

1.3 局部结构的分析

1.4 切屑分布的评价指标

 图 3 切屑分布的示意图 Figure 3 A schematic diagram of the pile distribution

2 模拟结果和讨论 2.1 刻划速度对刻划效果的影响

 图 4 不同刻划速度下的切屑分布 Figure 4 The pile distribution on different scratching velocity

 图 5 不同刻划速度下的俯视图与侧视图 Figure 5 The vertical view and lateral view piles distribution on different scratching velocity

2.2 针尖半径对刻划效果的影响

 图 6 不同针尖半径时的切屑分布 Figure 6 The pile distribution on different probe radius

 图 7 不同针尖半径时的俯视图与侧视图 Figure 7 The vertical view and lateral view piles distribution on different probe radius

 图 8 r=1.0 nm时探针的磨损和r=1.5 nm时探针的形变 Figure 8 The wears and tears of probe（r=1 nm）and the deform of probe（r=1.5 nm）

2.3 探针锥角对刻划效果的影响

 图 9 不同锥角时的切屑分布 Figure 9 The pile distribution on different probe wedge angles

 图 10 不同锥角时的俯视图与侧视图 Figure 10 The vertical view and lateral view pile distribution on different probe wedge angles

3 结　论

（1）刻划速度对沟槽表面的影响不大。为了使切屑更少地留于基底表面，刻划速度应当小于0.3 nm/ps或大于等于1.5 nm/ps。

（2）当针尖半径 $r {\text{≤}} 1\;{\rm nm}$ 时，探针会发生磨损；当 $r {\text{≥}} 1.5\;{\rm nm}$ 时，探针发生弹性形变。为了达到最佳的刻划效果（使b最大），针尖半径应取2.0～3.5 nm。

（3）较大的锥角有利于减少基底表面的切屑分布。

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