Ajit Balakrishna

Plasma-Profile Control Using External Circuit in a Capacitively Coupled Plasma Reactor

Very high frequency (VHF) capacitively coupled plasma (CCP) sources offer several benefits, including low plasma potential, high electron density, and controllable dissociation. However, standing electromagnetic waves can make the spatial structure of VHF plasmas a sensitive function of operating conditions and reactor geometry. This paper discusses how the profile of VHF CCPs can be controlled using an external circuit that modifies the electrical boundary conditions. A 2-D plasma model with external circuit has been used for this study, where networks of passive circuit elements can be connected to different electrodes in the reactor. Plasma simulations have been performed for several combinations of capacitors and inductors. It is found that the external circuit can be used to change the radio-frequency current return path, thereby modifying the plasma profile. In general, the plasma is pulled toward the electrode with an inductive impedance and pushed away from the electrode with a capacitive impedance. These changes in plasma profile are related to the relative voltage and their phase on different electrodes.

Model for a transformer-coupled toroidal plasma source

A two-dimensional fluid plasma model for a transformer-coupled toroidal plasma source is described. Ferrites are used in this device to improve the electromagnetic coupling between the primary coils carrying radio frequency (rf) current and a secondary plasma loop. Appropriate components of the Maxwell equations are solved to determine the electromagnetic fields and electron power deposition in the model. The effect of gas flow on species transport is also considered. The model is applied to 1 Torr Ar/NH3 plasma in this article. Rf electric field lines form a loop in the vacuum chamber and generate a plasma ring. Due to rapid dissociation of NH3, NHx+ ions are more prevalent near the gas inlet and Ar+ ions are the dominant ions farther downstream. NH3 and its by-products rapidly dissociate into small fragments as the gas flows through the plasma. With increasing source power, NH3 dissociates more readily and NHx+ ions are more tightly confined near the gas inlet. Gas flow rate significantly influences the...

Modeling of Plasma Processing Equipment—Current Trends and Challenges

Plasma modeling is a critical technology for the design of industrial plasma processing systems. Plasma processes are increasingly being extended to the sub‐20 mTorr regime in the microelectronics industry, requiring accurate plasma models in the low pressure regime. Simultaneously, economic considerations are imposing stringent requirements on plasma uniformity over large substrates and plasma modeling is expected to address these uniformity challenges. These trends are necessitating good theoretical understanding in low temperature plasmas of (a) kinetic phenomena at low pressures and (b) the complex interplay between plasma, electromagnetic, chemical and fluid dynamics phenomena in three dimensions. Several fluid and particle‐in‐cell models are used in this paper to address issues of importance to the design and use of plasma etching and deposition systems.

SiO2 etching in an Ar/c-C4F8/O2 dual frequency capacitively coupled plasma

SiO2 etching in an Ar/c-C4F8/O2 dual frequency (13.56 and 60 MHz) capacitively coupled plasma is examined in this paper. Experiments were done in a dilute mixture of c-C4F8/O2 in Ar for a wide range of conditions (low frequency power, c-C4F8 flow rate, O2 flow rate, total flow rate, and gas pressure), and the SiO2 etch rate was measured at multiple locations on 300 mm wafers. A two-dimensional hybrid fluid-kinetic plasma model was used to understand the experimental observations. A surface coverage based etch mechanism was found to best capture the experimental results over the range of conditions considered. In this mechanism, the SiO2 surface gets partially covered by a fluorocarbon thin film, and SiO2 is etched by energetic ions in the presence of these fluorocarbons. Conditions that enhance fluorocarbon coverage such as higher c-C4F8 flow rate or lower O2 flow rate lead to higher SiO2 etch rate. Many relevant quantities such as the fluxes of ions and neutral radicals to the wafer and ion energy sensit...

Plasma profile control using external circuit in capacitively coupled plasma reactors

Multi-frequency capacitively coupled plasma (CCP) sources in the range of 2-180 MHz are widely used for materials processing in the semiconductor industry. Very high frequency (VHF) sources bring various benefits including low plasma potential, high electron density, and controllable dissociation. Sources in the low frequency (LF) and high frequency (HF) regimes provide high ion energy with different ion energy distributions. Multi-frequency CCP reactors have been developed to combine the benefits of both frequency regimes. However, plasma structure can be difficult to control over a wide range of operating conditions (a few mTorr to several hundred mTorr, hundred to several thousand Watts, electropositive and electronegative chemistries). This paper discusses how the plasma structure can be controlled using external circuit impedance that modifies the plasma boundary conditions. A 2-dimensional plasma model with external circuit has been developed and used for this study. The plasma model considers conservation of charge species densities, momentum and energy along with the full set of Maxwell equations. The external circuit is a network of passive circuit elements connected to different electrodes. Plasma simulations have been performed for various external circuit configurations using combinations of capacitors and inductors. We found that the plasma structure and profile can be controlled by changing the current return path impedance. In general, the plasma is pulled towards the electrode with an inductive impedance and pushed away from the electrode with a capacitive impedance. When only capacitors are used, the plasma moves towards the electrode with lower impedance. As a result, the ion flux profile to the electrode can be controlled using the external circuit impedance. Electron density measurements using the resonance cavity method are used to verify that the electron density profile can indeed be modified using the external circuit impedance.