摘要
容性耦合等离子体(CCP)被广泛地应用于微电子制造工业的刻蚀、薄膜沉积和其它表面处理工艺中。在刻蚀工艺中,反应腔室内的等离子体密度和轰击到电极上的离子能量起着极其重要的作用,因为等离子体密度直接影响刻蚀速率,离子能量则影响刻蚀的选择性及对产品的损伤,实现对等离子体密度和离子能量的独立控制可以间接的调制刻蚀率、选择性等工艺参数。双频CCP(DF-CCP)被认为是一种解决等离子体密度和离子能量分离控制的有效方法,可以产生大面积均匀等离子体,并且通过调节高、低频的放电参数可以有效的控制等离子体密度、轰击到基片上的离子能量等关键物理参数。其结构简单、成本低,因而在新一代的刻蚀设备中得到了广泛应用。
等离子体必须经过鞘层才能与基片发生相互作用,因此,双频偏压容性耦合等离子体(DF-CCP)鞘层中的各物理参量及其物理过程对等离子体刻蚀工艺有着直接的影响,使得双频鞘层特性一直受到研究者们的关注。
在第二章中针对双频偏压等离子体鞘层建立了一维流体力学模型。该模型假设离子满足流体力学方程,电子遵循玻尔兹曼关系,瞬时电场由泊松方程确定;并采用等效电路方法建立了电流平衡方程,来自洽地确定鞘层厚度和鞘层电势之间的关系。
本章分别研究了在双射频偏压和双脉冲偏压条件下的鞘层特性及参数,如鞘层内的离子密度,电场的空间分布以及充电效应。结果表明:等离子体鞘层特性受高、低频共同调制,等离子体密度主要受高频调制,离子能量主要受低频调制。而且在双脉冲条件下绝缘基片上积累的电荷,明显低于双射频条件下的正电荷。
在第三章针对电极的位形为一个台阶或一个柱形圆槽情况,建立了二维的流体力学模型,研究了等离子体鞘层中各物理参量的变化规律。其中描述离子的流体力学方程采用通量修正算法(FCT)方法来解,二维的泊松方程用超松弛高斯—赛德尔迭代方法求得。计算的结果表明:极板电位与鞘层的厚度受到高低频电源的共同调制,鞘层内同一位置的电位降随着放电气压的增加而减小,且在电极附近等势线的形状与电极的位形相似,即所谓的“Plasma molding”效应;等离子体的密度也有与电位降类似的变化规律,而且电场和离子流速在电极位形发生变化的台阶内侧很强,且变化很快。
Capacitively coupled plasmas (CCPs) are used extensively in the macro-electronics manufacturing as etching, deposition and other surface treatment devices; for etching processing, plasma density and ion energy bombarding the electrodes are crucial issues because the high plasma density leads to high etch rates while ion bombardment energy is vital for improving selectivity and avoiding dielectric damage. Controlling the plasma density and the ion energy independently can modulate etch rates, etching selectivity and other processing parameters indirectly. Dual frequency CCP has been considered as an effective tool for the independent control of the plasma density and ion bombarding energy, since it can generate large area uniformly plasma. The plasma parameters such as plasma density, ion bombardment energy can be effectively controlled by modulating the discharge parameters of low and high frequency sources. For its simple configuration and low cost, it has been widely used in the next generation plasma etching equipments.
Ions must pass through the sheath before reacting with the wafer; hence, plasma etching processing is determined directly by physical parameters and process in the sheath, so the sheath characteristics is extensively studied by many research workers.
In Chapter 2 , a one dimensional fluid model is developed for plasma sheaths. Ion density and ion drift velocity are described by cold fluid equations, electron density is given by Boltzmann relation and instantaneous electric field is determined by Poisson's equation; the relation between the sheath thickness and potential is determined self-consistently by the current balance equation which is obtained from the equivalent circuit model.
We studied the characteristics and parameters of plasma sheath driven by dual radio frequency and dual pulse respectively, such as plasma density, sheath electron spatiotemporal distribution and charging effect. The results show characteristics of the sheath are modulated by low and high sources; plasma density is modulated by high source but ion energy modulated by low one independently; and the surface charge density on the insulating substrate with the pulse bias is obviously lower than that of the radio frequency bias.
In Chapter 3, a two dimensional model is also developed for the case of an electrode with a step or cylindrical hole, physical parameters of plasma sheath is studied under the different discharge pressure; The fluid equations describing the ion were solved by Flux Corrected Transport (FCT) method, and two dimensional Poisson equation is solved by the Successive-Over-Relaxation (SOR) Gauss-Seidel iterative method. The results show the electrode potential and sheath thickness were controlled by both low-frequency and high-frequency sources, the potential at the same position in the sheath decreased as the pressure increased, while the shape of potential-isoline near the electrode is similar with the shape of electrode, which is also referred as "Plasma molding" effect; ion density in sheath has the similar profiles with potential; radial electric field and ion velocity at inner side of hole are strong and changed significantly in space.
引文
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