Session ER4: Low Pressure Capacitively Coupled Plasmas

Electron power absorption mode transitions in low-pressure capacitively coupled plasmas in gas mixtures

The electron power absorption and excitation dynamics are studied by Phase Resolved Optical Emission Spectroscopy (PROES) measurements combined with Particle-in-Cell/Monte Carlo Collisions (PIC/MCC) simulations in low-pressure capacitively coupled plasmas (CCPs) operated in gas mixtures. Results of the PROES measurements performed in a geometrically symmetric CCP reactor show good qualitative agreement with the PIC/MCC simulation results in a wide parameter range in Ne-O₂ and Ar-O₂ mixtures. Various emission and excitation patterns develop at different pressures, driving frequencies and mixing ratios of the two gases. The mechanisms behind the formation of these patterns are revealed based on the Boltzmann term analysis. The transitions between different discharge operation modes and electron power absorption modes are examined. Significant changes in the excitation patterns are generally considered to be predictive of transitions of the dominant electron power absorption mode and discharge operation mode. The results show that it is not straightforward to infer the power absorption mode transitions based on the spatio-temporal distribution of the excitation rate alone as the same electron power absorption mechanisms could be associated with excitation patterns of significantly different characteristics, which indicate discharge operation in different modes.   

The effects of different boundary surface materials on the electron power absorption dynamics in capacitive RF plasmas

Low temperature plasmas are used to modify boundary surfaces, but boundary surfaces affect the plasma, too. Via material specific probabilities for particle absorption, reflection, emission, and conversion such surfaces can change process relevant plasma parameters such as flux-energy distribution functions of electrons and ions. In this work, we demonstrate experimentally how the presence of different wafer materials in capacitive radio frequency plasmas can change the mode of electron power absorption via their different ion induced secondary electron emission coefficients under otherwise identical discharge conditions. Exposing two electrodes made of different materials to the plasma at the same time is found to induce a plasma asymmetry in an otherwise symmetric reactor and the simultaneous presence of different modes of electron power absorption at each electrode. This leads to the generation of a DC self bias as a consequence of this Material Asymmetry. These experimental findings are compared to computational results obtained from Particle in Cell simulations to gain further insights.   

A hybrid surface cavity microwave source with a capacitively coupled plasma for plasma uniformity control

CCP discharges have emerged as the workhorse technology in a wide variety of applications, including fabrication of nano-devices with very stringent requirements. CCP discharges are traditionally electrostatically driven, but recent very high-frequency (VHF) operation has magnified the importance of the physics of coupling of electromagnetic waves in the plasma. Such coupling is still poorly understood despite nearly two decades of research in this area. This is mainly due to the scarcity of space and time-resolved experimental measurements of key variables, and unresolved physics representation in computational models of very high frequency capacitively coupled plasmas (VHF-CCP). In this study, common problems with CCP discharge uniformity will be highlighted using argon plasma under different system pressures. Furthermore, certain additions to the CCP geometry are investigated to observe its effects on its domain. For example, the addition of a vane type structure along with the electrode adds further tunable paramenters, improves electric field and plasma coupling. The addition of this surface wave cavity can make traditional CCP’s scalable which allows large-scale domains. However, the scattered nature of the electric field in the waveguide and, by extension, the CCP gas domain may result in heavily varying species densities.   

Coupled ion-ion oscillation and periodic emission pattern in electronegative capacitively coupled radio-frequency plasmas

Spatiotemporal  dynamics in a capacitively coupled electronegative plasma discharge (Ar/O2 mixture) generated using a combination of 13.56 MHz sinusoidal voltage and a low frequency of a few hundred kHz tailored voltage is examined both experimentally and numerically. A periodic electron impact excitation pattern with a frequency of a few MHz has been observed by phase-resolved optical emission spectroscopy (PROES). Their formation is linked to the coupled oscillation between the positive and negative ions. The onset of this unique oscillation is set off by the electron motion triggered at the edge of the low frequency (LF) waveform, followed by the separation between positive/negative ions, and then maintained by the long-lasting ion-ion coupled motion. The established modulation of the electric field and the electron density by the ion motion generate periodic patterns in the excitation of metastable species.  It is also shown in our particle in cell/Monte Carlo collision (PIC/MCC) simulations that the frequency of this ion-ion oscillation is determined only by the ion plasma frequency and its amplitude is determined by the peak-to-peak voltage of the LF and plasma electronegativity, independent of the LF waveform and frequency.  The emission patterns from the experiments with different waveforms and frequencies of the LF voltage show a good agreement with the simulation results.   

Two-dimensional kinetic modeling of a weakly magnetized Capacitively Coupled Plasma Discharge in cylindrical geometry.

Capacitively Coupled Plasma (CCP) discharges are commonly used in the semiconductor industry to etch and deposit materials for microelectronics components. The processing rates and uniformity of the wafer depend on key parameters such as the ion flux, ion energy distribution function (IEDF), and plasma homogeneity. The non-Maxwellian nature of the IEDF requires a kinetic treatment, which can be achieved with Particle-In-Cell (PIC) simulations.

In this work, we develop a procedure to control the plasma uniformity and its dynamics using a weak magnetic field with a 2D cylindrical axisymmetric model. The present investigation takes leverage of PIC modeling and uses the explicit EDIPIC-2D code [1]. A detailed analysis of the sheath structure, the ion flux, and IEDF at the wafer will be performed and compared to prior 1D models [2].

 

¹ https://github.com/PrincetonUniversity/EDIPIC-2D

² S. Patil, S. Sharma, S. Sengupta, A. Sen, and I. Kaganovich, “Electron bounce-cyclotron resonance in capacitive discharges at low magnetic fields,” Phys. Rev. Research 4(1), 013059 (2022).   

Discharge properties of a cylindrical capacitively coupled plasma discharge with an axisymmetric magnetic field

Radio-Frequency Capacitively Coupled Plasma (CCP) discharges in semiconductor device manufacturing are indispensable for surface modifications, including PECVD and plasma etching. In such discharges, enhanced efficiency of the plasma system and control over vital discharge parameters are crucial for critical process conditions. We present a novel CCP device with a 13.56 MHz powered cylindrical electrode, sets of grounded annular rings, and grids on its axial ends. The axial magnetic field (B) generates E × B drifts in the azimuthal direction, supporting enhanced and homogeneous plasma density and providing uniform surface modification. The discharge characteristics and bulk plasma diagnostic, including the Energy Distribution Functions of electrons (EEDF) and ions (IEDF), are performed using an inline IV sensor, RF-compensated Langmuir probe, and Hiden EQP-300, respectively. The experimental results show a range of B-fields where the discharge is highly efficient with lower electron temperature. Outside this range, the plasma density drops, followed by an increase in the electron temperature. The behavior is attributed to the transition from geometrical asymmetry to magnetic field-associated symmetry due to reduced radial losses. The EEDF shows a transition from bi-Maxwellian for B=0 to Maxwellian at intermediate B and a bi-Maxwellian at high B. The B-field's effect on the IEDF, a plausible explanation for the observations, and potential applications of the device are highlighted.