Two-dimensional Fluid Simulation Of The RF Bias Plasma Sheath Characteristics

Plasma etching is one of the most crucial processes in MEMS(Micro Electro-Mechanical Systems) manufacturing and which is attracting more and more people's attention.Because it can be carried out to achieve the processing requirement,e.g.high etching rate,high anisotropy, high aspect ratio,high selectivity and lower dielectric damage.Radio-frequency capacitive coupling discharge as one of the high-frequency discharge method,since it can generate large area uniformly plasma,more importantly,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,and its simple structure,low cost,all these advantage made RF capacitively coupled plasma(CCP)be used extensively in the semiconductor etching and deposition of thin films as well as surface modification.In the etching process of the wafer,due to the accessing of RF power sources,a RF bias sheath will be formed near the powered etectrode,and most of the ion energy is obtained by the acceleration of electric field in the sheath and which is far more than the ion thermal energy,furthermore,ion energy distributions directly determines the rate of etching and the processing quality.So it is important to study the characteristics of the sheath.In MEMS manufacturing,the processed substrate always exhibit a nonplanar surface structure,When the surface features on the substrate were extremely small compared to the sheath thickness,it is usually assuming that the sheath profile is uniform in the direction parallel to the plate,the sheath models mainly focused on studying the sheath in one dimension,however,when the size of features on the substrate can be comparable to or larger than the sheath thickness,the sheath profile in the direction parallel to the substrate would not be neglected any more,in such cases,we must use the 2D(or multi-dimensional) model to accurately describe the physical characteristics of the sheath.In chapter 2,a 2D fluid model is developed to study the sheath characteristics over the electrode with a circular trench,we simulated the impacts of the aspect ratio of the trench on the RF bias sheath characteristics.The results show that the sheath profile in the radial direction would be nonuniform anymore,in such cases,the sheath profile always tend to adapt the contours of the electrode,which is the plasma molding effects.In the two sidewalls of the trench,internal and external,sheath potential and electric field perform an obvious asymmetry. As the increase of aspect ratio,the sheath potential drop and sheath thickness decrease. As well as we know,CCP driven by dual RF(DF) sources makes it possible to obtain the desired independent control of plasma density in the reactor and the ion energy impinging on the substrate.The high-frequency(HF) source mainly control the plasma density, low-frequency(LF) source control the ion energy.In chapter 3,we still adopt 2D fluid model studied the DF sheath characteristics near the electrode with a cylindrical hole.Sheath potential and electric field are calculated and compared for different LF frequencies,power and discharge pressure.Fixed HF parameters,as the increase of LF frequency or the decrease of the LF power,the sheath potential drop and sheath thickness is thinner,the plasma molding effect seems to be more significant.Fixed the parameters of the DF source,as the rise of the pressure the sheath potential increase and the sheath is thicker.