低气压容性射频等离子体鞘层特性研究/Research on Low-Pressure Capacitive Radio Frequency Plasma She

2018-08-19 10:04:20

The RF potential plasma sheath



摘要
射频(RF)等离子体加工技术已经广泛地应用于微细制造领域,如集成电路制造中的等离子体干法刻蚀、微结构制造中的体硅刻蚀、薄膜淀积以及溅射工艺等,成为现代高新制造技术领域的关键技术。在等离子体加工中,工件表面附近的等离子体状态和性质是非常重要的,当等离子体与固体表面接触时,将在等离子体和表面交界处形成由Debye屏蔽产生的等离子体鞘层。在鞘层中,电子数密度和电势有一个急剧的指数降落,形成一个负电位的强电场,使鞘层中的离子获得加速,并以相当高的能量轰击到材料的表面上,在等离子体加工中起着关键性的作用。射频鞘层中,由于射频电场的瞬变和鞘内电子的振荡,使得鞘层内存在着很强的非线性过程。
在Lieberman的射频鞘层模型基础上,研究了低气压容性射频等离子体鞘层特性。在鞘层和鞘前准电中性过渡区中,研究了影响离子运动的鞘层电场。考虑Bohm速度对应的鞘前电势差,并与鞘内时间平均的电势合并,推出了总电势差的表示式。将鞘前和鞘内电势差推广到材料表面,得到了从等离子体内部到表面的电势差表示式。利用时间平均的鞘层电势差,计算了离子经过RF鞘层电场加速后获得的平均动能,以及到达材料表面时,离子轰击表面的能量、能量通量和动量。在低气压容性射频等离子体实验中,测量了通过功率电极极板的RF电流、RF电压有效值、极板间DC自偏置电压。通过数据处理,把RF电流有效值换算成单位面积上电流密度的幅值,把RF电压有效值换算成电压幅值,并与测量的DC自偏置电压一起计算功率电极表面附近的鞘层电势差。实验结果与理论分析进行了比较,呈现较好的吻合,为理论分析的正确性提供了证据。在此基础上分析了离子能量、能量通量、动量与RF电流密度、等离子体浓度、鞘电子温度等参量的关系,以及各参量对离子轰击材料表面的能量、能量通量及动量的影响。
论文的创造性工作包括:
(1)提出了Lieberman的RF鞘层模型与射频电场下Bohm鞘前模型的合并,得到了从等离子体内部到鞘层内的总鞘层电场模型,以及包括鞘前和鞘内电势在内的鞘层电势差的表示式;
(2)将包括鞘前和鞘内电势的鞘层电势差推广到材料表面,推出了从等离子体内部到材料表面的鞘层电势差与RF电流密度、等离子体浓度以及鞘电子温度的关系;
(3)提出了基于时间平均鞘层电势的离子平均动能模型,推出了离子到达被加工材料表面时的动能与RF电流密度、等离子体浓度以及鞘电子温度的关系,分析了各参量对离子能量的影响;
(4)提出了基于时间平均鞘层电势的离子能量通量模型,推出了离子到达被加工材料表面时的能量通量与RF电流密度、等离子体浓度以及鞘电子温度的关系,分析了各参量对离子能量通量的影响;
(5)提出了基于时间平均鞘层电势的离子平均动量模型,推出了离子到达被加工材料表面时的动量与RF电流密度、等离子体浓度以及鞘电子温度的关系,分析了各参量对离子动量的影响;
(6)在低气压容性射频等离子体实验中,测量了功率电极表面附近的鞘层参数,利用Coburn公式,得到了鞘层电势差与电流密度之间的关系数据;
(7)通过比较射频等离子体鞘层实验数据和理论模型,验证了理论分析的正确性。

Abstract
In the micro-fabrication processes such as dry etching for integrated circuit manufacturing, solid silicon etching for micro-structure fabrication, thin film depositing, and sputtering, etc., radio frequency (RF) plasma manufacturing has been extensively used and has become a key technology in the modern high-tech industry. In the plasma processes, the status and characteristics of the plasma near the material surface are of great importance. When plasma gets in touch with solid surface, a sheath caused by Debye screen effect appears between them. In this plasma sheath, a sharp exponential decline of electron density and potential occurs, and a strong electric field is formed, in which ions are accelerated until they strike the material surface with certain high energy. The ion's energy impinging on the surface has significant effect in plasma processes. In the RF sheath, many phenomena are strongly non-linear because of the instantaneous RF field and the oscillating electrons in the sheath.
On the basis of Lieberman's RF sheath model, the characteristics for low-pressure capacitive radio frequency plasma sheath are investigated. The electric fields by which the ions are accelerated are studied in the sheath and the presheath quasi-neutral areas. The presheath potential drop corresponding to Bohm velocity is considered and combined with the inner sheath time-average potential. The equation for total potential difference is derived. After extending the presheath and inner sheath potential difference to the surface, the equation for the potential difference from the plasma bulk to the material surface is obtained. Using the time-average sheath potential difference, the average ion kinetic energy gained by the acceleration of RF sheath field is calculated and the energy, energy flux, momentum with which ions impinge on the material surface are obtained. In the low-pressure capacitive radio frequency plasma experiment, the effective RF current flowing through the powered electrode, the effective RF voltage applied on the powered electrode, and the DC self-bias between the electrodes are measured. By data processing, the effective value of RF current is converted to the amplitude of RF current density per unit area; the effective value of RF voltage is converted into amplitude, which is combined with the measured DC self-bias to calculate the sheath potential difference near the powered electrode surface. The experimental results are compared with the analytical model, which shows agreement, and provide evidence for the validity of the analytical model. Based on these, the relationships between the ion energy, energy flux, momentum and the parameters, including RF current density, plasma density, and sheath electron temperature, are derived. Effects of the parameters on the ion energy, energy flux, momentum impinging on the material surface are analyzed.
The creative works are as follows:
(1) The combination of Lieberman's RF sheath model with the Bohm presheath model in RF field is proposed. The model for total sheath field from the plasma bulk to the inner sheath is obtained, and the equation for sheath potential difference including the presheath and inner sheath potential is derived.
(2) The sheath potential difference including the presheath and inner sheath potential is extended to the surface. The relationship between the sheath potential difference from the plasma bulk to the material surface and the parameters, including RF current density, plasma density, and sheath electron temperature, is derived.
(3) The model for average ion kinetic energy based on time-average sheath potential is proposed. The relationship between ion energy when reaching the material surface and the parameters, including RF current density, plasma density, and sheath electron temperature, is derived. The effects of the parameters on the ion energy are analyzed.
(4) The model for ion energy flux based on time-average sheath potential is proposed. The relationship between ion energy flux when reaching the material surface and the parameters, including RF current density, plasma density, and sheath electron temperature, is derived. The effects of the parameters on the ion energy flux are analyzed.
(5) The model for average ion momentum based on time-average sheath potential is proposed. The relationship between ion momentum when reaching the material surface and the parameters, including RF current density, plasma density, and sheath electron temperature, is derived. The effects of the parameters on the ion momentum are analyzed.
(6) In the low-pressure capacitive radio frequency plasma experiment, the sheath parameters near the powered electrode surface are measured. With Coburn formula, the related data for sheath potential difference and current density are obtained.
(7) By comparing the experimental data from the RF plasma sheath experiment with the analytical model, the validity of the theoretical analysis is verified.