Structure of the velocity distribution of sheath-accelerated secondary electrons in an asymmetric RF-dc discharge

Abstract

Low-pressure capacitively-coupled discharges with additional DC bias applied to a separate electrode are utilized in plasma-assisted etching for semiconductor device manufacturing. Measurements of the electron velocity distribution function (EVDF) of the flux impinging on the wafer, as well as in the plasma bulk, show a thermal population and additional peaks within a broad range of energies. That range extends from the thermal level up to the value for the "ballistic" peak corresponding to the bias potential. The non-thermal electron flux has been correlated to alleviating the electron shading effect and providing etch-resistance properties to masking photoresist layers. "Middle-energy peak electrons" at energies of several hundred eV may provide an additional sustaining mechanism for the discharge. These features in the electron velocity (or energy) distribution functions are possibly caused by secondary electrons emitted from the electrodes and interacting with two high-voltage sheaths: a stationary sheath at the DC electrode and an oscillating, self-biased sheath at the powered electrode. Since at those energies the mean free path for large-angle scattering (momentum relaxation length) is comparable to, or exceeds the size of the discharge gap, these "ballistic" electrons will not be fully scattered by the background gas as they traverse the inter-electrode space. We have performed test-particle simulations where the features in the EVDF of electrons impacting the RF electrode are fully resolved at all energies. An analytical model has been developed to predict existence of peaked and step-like structures in the EVDF. These features can be explained by analyzing the kinematics of electron trajectories in the discharge gap.Step-like structures in the EVDF near the powered electrode appear due to accumulation of trapped electrons during a part of the RF cycle and their subsequent release. Trapping occurs when the RF sheath voltage exceeds the applied bias, and is decreasing. The step structures are formed by secondary electrons originating from the DC-biased surface (which also produce a peak near the energy equal to the bias potential). Additional peaks, at lower energies, are formed by the electrons emitted from the RF electrode and eventually escaping to it. The latter electrons can be grouped according to the number of bounces between the sheaths during their residence time in the discharge. Each of such groups may give rise to an individual peak in the distribution. The trap-and-release theory developed in this paper provides a convincing explanation for the observations of the ballistic and "middle energy peak" electrons measured in experiments.

The 2017 Plasma Roadmap: Low temperature plasma science and technology

“…Plasma chemistry and interactions with boundaries are often the responsible mechanisms for producing desired outcomes in industrial devices; so the ability to control them can provide a basis for improving the performance of existing devices, as well as for the development of new devices. Examples of methods being explored in this area include tailored RF waveforms [159], externally biased electrodes [160] and electron emitting surfaces [161].…”

Generation of anomalously energetic suprathermal electrons by an electron beam interacting with a nonuniform plasma

“…A similar mechanism may explain enhanced energetic tails in the energy spectrum of electrons in aurora [9,20,21]. Other mechanisms invoked to explain the EVDF structure include phase-bunching, kinematic effect of electrons being trapped between rf and dc potentials and released towards the rf electrode during short period of time when rf voltage is small compared to the dc voltage [22].…”

Spatial symmetry breaking in single-frequency CCP discharge with transverse magnetic field

“…The metastable reactions are not considered because of the ambient low pressure. At high voltages secondary electrons generated by either ion or electron-induced emission can modify the discharge 47 . However, for the sake of simplicity in this paper we only focus on the physics of the rf sheath and its modification by a weak magnetic field and neglect the generation of secondary electrons.…”