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Abstract
A one-dimensional fluid/Monte Carlo hybrid model was used to quantitatively study the secondary electron effect on sustaining the discharge by examining the ionization induced by bulk electrons and secondary electrons under different external discharge parameters. The results indicate that as the voltage increases, secondary electrons gain more energy from the stronger electric field. Therefore, the ionization region induced by secondary electrons expands and the ionization rate becomes comparable to and even exceeds that of bulk electrons. As the pressure increases, secondary electrons collide with neutrals sufficiently, thus their contribution to the plasma generation becomes pronounced and eventually they dominate the discharge. Besides, the distribution of the secondary electron ionization rate varies from flat to saddle-shape, due to the energy loss at the discharge center at higher pressures. Finally, when the discharge gap expands, the electron density calculated in the case without secondary electrons increases linearly, whereas the value first increases and then decreases in the model with secondary electrons taken into account. The results obtained in this work are important for improving the high aspect ratio etching process by secondary electrons.
Highlights
Coupled plasmas (CCP), one of the most widely used plasma sources in the industry, play an important role in modern plasma processing technologies.1 In the etching process, especially as the aspect ratio of the etching structure increases, abundant positive ions accumulate at the bottom of the trench, whereas, negative charging takes place at the top of the feature due to the isotropic electron flux
Since the generation of secondary electrons is influenced by various external parameters, the secondary electron effect has been examined under different voltages, gas pressures and discharge gaps
(1) Voltage effect: Since the energy obtained by secondary electrons is low due to the weak electric field at low voltages, secondary electrons only have a negligible influence on the plasma density, and the discharge is mainly sustained by bulk electrons
Summary
Coupled plasmas (CCP), one of the most widely used plasma sources in the industry, play an important role in modern plasma processing technologies. In the etching process, especially as the aspect ratio of the etching structure increases, abundant positive ions accumulate at the bottom of the trench, whereas, negative charging takes place at the top of the feature due to the isotropic electron flux. At the beginning of each call to the eMCs, the pseudoparticles, which represent secondary electrons, are placed at the walls of the chamber, with a Maxwellian velocity distribution of 4 eV, and the particle number is proportional to the ion flux. When these particles bombard the walls or their energy is lower than the threshold, they will be removed. The simulation starts from the full fluid module (i.e., including the electron energy equation), which runs for 10 rf cycles to produce the initial electric field Ez. Subsequently, Ez is input into the eMCs module, which contains bulk electron simulation and secondary electron simulation. The plasma density, EEDF, as well as the ionization rate, are produced to investigate the secondary electron effect on sustaining capacitively coupled discharge
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