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为了研究杂质辅助离子束对与杂质靶不同距离处蓝宝石的刻蚀作用,采用自制的离子束刻蚀系统进行了实验。采用微波回旋共振离子源产生等离子体。微波回旋共振离子源工作示意图如图1所示。在放电室内,当电子回旋频律和沿磁场传播的右旋圆极化微波频率相等时产生共振,此时电子在微波电场中被不断同步加速而获得的足够大能量,碰撞作气体电子使其分离,实现等离子体放电,形成高密度的ECR低温等离子体。Ar+等离子束用加速栅平行引出,实现对基底的溅射刻蚀。该离子源口径为Φ120 mm,离子源微波功率0~400 W可调,离子束能量为200~2 000 eV,束流密度为0~3 000 μA/cm2。
不锈钢是杂质辅助离子束诱导纳米结构研究中常用的杂质之一,因此实验也选择不锈钢作为杂质靶材料。图2为此次实验中所用的杂质靶的示意图。该杂质靶是一个垂直高度为8.66 mm、边长为10 mm、厚度为1 mm、长度为30 mm的屋脊结构,样片在杂质靶中的放置位置如图中A、B、C、D所示,离子束入射方向如图中黑色箭头所示。
采用布鲁克生产的Innva型多模式原子力显微镜(Atomic Force Microscope, AFM)来观察样品表面的形貌变化,Taylor Surf CCI 2000白光干涉表面测量仪测量蓝宝石样片表面的粗糙度;利用功率谱密度(Power Spectral Density, PSD)来观察纳米结构有序性;选择X射线光电子能谱(X-ray Photoelectron Spectroscopy, XPS)对样品表面的化学成分进行了表征,利用傅里叶红外光谱仪对距杂质靶不同距离处,刻蚀后的蓝宝石样片进行透射率测量。
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实验在室温下进行,本底真空2×10−3 Pa,工作真空2.5×10−2 Pa,加速电压350 eV,离子源微波功率为345 W,刻蚀气体选用纯度为99.999%的Ar气,充气流量由气体流量计控制为7.0 sccm,设置的离子束参数如表1所示。
表 1 离子束参数
Table 1. Ion beam parameters
Parameters Value Angle of incidence/(°) 65 Ion beam current density/μA∙cm−2 487 Energy of incident ion beam/eV 1000 Erosion duration/min 60 在实验中,基片选用双面抛光C向蓝宝石,样品如图3所示。样片安装在具有水冷装置的工件台上,该工件台可绕自身轴旋转,离子束入射角度相对于样片表面法线可实现0~90°可调。
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Bradley-Harper (BH)理论模型[16]是建立在Sigmund溅射理论的基础上提出来的,描述离子束刻蚀非晶固体表面自组织纳米结构形成的物理机制。由于固体表面局部曲率原子溅射产额的不同,从而导致溅射速率不同,因此固体表面形成了调制性的自组织纳米结构。
Bradley和Harper推导了表面形貌演化的线性方程:
$$ \frac{{\partial h}}{{\partial t}} = - {v_0} + {v_x}\frac{{{\partial ^2}h}}{{\partial {x^2}}} + {v_y}\frac{{{\partial ^2}h}}{{\partial {y^2}}} + K{\nabla ^4}h + \eta \left( {x,y,t} \right) $$ (2.1) 式中:v0为无其他影响时的刻蚀速率;K为表面扩散导致的弛豫率;
$\eta (x,y,t)$ 为高斯随机噪音;vx,vy为刻蚀过程中产生的有效表面张力,与离子入射角有关。BH模型认为由于表面热扩散是在温度较高的条件下才能引起表面平滑,在高温条件下,BH模型却可以解释条纹的形成机制,但是无法解释在低温条件下形成的光滑条纹。
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经过大量实验研究和模型定量分析后发现,在离子束溅射过程中,不仅仅只存在表面热扩散这一种平滑机制,表面粘滞流也是平滑机制很重要的影响因素,对于表面纳米结构的形成也起到了重要作用。
表面粘滞流是指固体表面的原子因为存在压差而发生的流体性运动,出现在非晶体材料的表面[17]。其数学表达式为:
$$ \frac{{\partial h}}{{\partial t}} = - J\varOmega \frac{{\gamma {\Delta ^3}}}{{{\eta _r}}}{\nabla ^4}h $$ (2.2) 式中:Δ表示存在粘滞流的表面薄层厚度;ηr表示粘滞系数;γ表示表面能;Ω表示原子体积。
加入了表面粘滞流因素,修正的BH模型可表示为:
$$ \frac{{\partial h}}{{\partial t}} = - {v_0} + {v_x}\frac{{{\partial ^2}h}}{{\partial {x^2}}} + {v_y}\frac{{{\partial ^2}h}}{{\partial {y^2}}} + \left( {J\varOmega \frac{{\gamma {\Delta ^3}}}{{{\eta _r}}} + K} \right){\nabla ^4}h + \eta \left( {x,y,t} \right) $$ (2.3)
Evolution of sapphire surface nanostructure with distance of impurity target
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摘要: 使用微波回旋共振离子源刻蚀蓝宝石(C向)表面,引入金属不锈钢杂质,研究了不同靶距处蓝宝石表面自组织纳米结构的演化规律及光学性能。采用原子力显微镜来观察样品表面的形貌变化,Taylor Surf CCI 2000白光干涉表面测量仪测量蓝宝石样片表面的粗糙度;选择X射线光电子能谱对样品表面的化学成分进行了表征。实验结果表明:当离子束能量为1000 eV,束流密度为487 μA/cm2,入射角度为65°,刻蚀时间为60 min,蓝宝石样片与杂质靶距离从1 cm增加到4 cm时,样片表面出现岛状结构并逐渐演变为连续的条纹结构。同时,自组织纳米结构随靶距增加,有序性增加,纵向高度逐渐减小,空间频率基本不变。刻蚀后样品表面的金属杂质残留很少,微结构的形成对蓝宝石具有增透作用。在离子束溅射过程中,岛状结构的出现促进了样品表面条纹纳米结构的生长,破坏了纳米结构的有序性。Abstract: Electron cyclotron resonance ion source has been employed to etch the surface of sapphire (C-cut), introducing metallic stainless steel impurities to investigate the evolution law and optical properties of the self-organized nanostructure on the sapphire surface at different target distances. The atomic force microscope was used to observe the morphological changes of the sample surface, the Taylor Surf CCI 2000 white light interference surface measuring instrument was used to measure the surface roughness; X-ray photoelectron spectroscopy was selected to characterize the chemical composition. The experimental results indicate that, with the ion beam energy of 1000 eV, the beam current density of 487 μA/cm2, the oblique incident angle of 65°, and the erosion duration of 60 min, the distance between the sapphire sample and the impurity target increases from 1 cm to 4 cm, island-like structures appear on the sample surface and gradually evolve into continuous ripple structures. At the same time, as the target distance increases, the orderliness of the self-organized nanostructures enhances, the longitudinal height gradually decreases, while the spatial frequency is unchanged. There are very few metal impurities on the etched sample surface. The appearance of microstructures has antireflection effect on sapphire. During the ion beam sputtering process, island-like structures promotes the growth of ripple nanostructures but destroys orderliness.
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表 1 离子束参数
Table 1. Ion beam parameters
Parameters Value Angle of incidence/(°) 65 Ion beam current density/μA∙cm−2 487 Energy of incident ion beam/eV 1000 Erosion duration/min 60 -
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