Plasma density and ion energy control via driving frequency and applied voltage in a collisionless capacitively coupled plasma discharge

Abstract

The dynamical characteristics of a single frequency low pressure capacitively coupled plasma (CCP) device under varying applied RF voltages and driving frequencies are studied using particle-in-cell/Monte Carlo collision simulations. An operational regime is identified where for a given voltage the plasma density is found to remain constant over a range of driving frequencies and to then increase rapidly as a function of the driving frequency. The threshold frequency for this mode transition as well as the value of the constant density is found to increase with an increase in the applied voltage. Over the constant density range, for a given voltage, the sheath width is seen to increase as a function of the increasing driving frequency, thereby changing the ion energy without affecting the ion density. Our parametric study thus indicates that the twin knobs of the applied voltage and driving frequency offer a means of independently controlling the density and the ion energy in a low pressure CCP device that may be usefully exploited for plasma processing applications.Single frequency capacitively coupled (CCP) plasma devices have been extensively employed in the plasma processing industry for applications like the etching and deposition of thin films 1 . A CCP device consists of two parallel electrodes between which a plasma discharge is struck by biasing the electrodes with a radio frequency power supply that is typically operated at a single frequency of 13.56 MHz. The bulk plasma is separated from the electrodes by the formation of space charge sheaths whose thicknesses oscillate at the driving frequency. The sheath and bulk plasma properties of the discharge are influenced by a number of factors such as the driving frequency, the applied voltage, the background neutral pressure, the gap between the electrodes and the relative areas of the electrodes. For material processing applications the plasma density and the ion energy are important parameters that have a significant impact on the quality and rate of etching/deposition processes. In low pressure discharges the ions gain energy mainly through their acceleration from the DC bias field within the sheath while the electrons gain energy through a stochastic heating process through their interaction with the high-voltage oscillating sheath 2-17 . The plasma density is controlled both by the applied voltage and the driving frequency. Typically it increases linearly with the applied voltage and quadratically with the driving frequency. In the traditional single frequency CCP device the plasma density and the ion energy cannot be changed independently since an increase in the RF power changes both the density of the plasma and the sheath voltage which affects the energy of the ions 1 . Such a limitation of the single frequency CCP has led to searches for alternative configurations as well as explorations of different parametric regimes. The dual frequency CCP has been a major development in this direca) e-mail: sarvesh@ipr.res.in tion and is now quite widely used ...

Electric field filamentation and higher harmonic generation in very high frequency capacitive discharges

“…At this driving frequency, we had previously observed strong electric field transients in the bulk plasma [19,21]. Furthermore, this frequency is well above the transition frequency where the transients emerging from one sheath interact with the opposite sheath [19,21]. The discharge voltage is now varied from 10 V to 150 V at a constant gas pressure of 5 mTorr.…”

Inducing locally structured ion energy distributions in intermediate-pressure plasmas

“…Structured IEDFs differ from unstructured distributions as they exhibit additional characteristic peaks, typically associated with the radio-frequency modulation of the sheath potential 14 .Previous works related to structured IEDFs in rf plasmas were primarily focused over the range from low to intermediate pressure discharges (0.65 -67 Pa, 5 -500 mTorr) at applied voltage frequencies of 13.56 MHz 14-17 and 27.12 MHz 18 . More recent work has demonstrated control of the IEDF in low-pressure (0.65 -1.3 Pa, 5 -10mTorr) plasmas through application of higher applied voltage frequencies in the range 12 MHz -100 MHz 12,19,20 . For applications involving higher-pressure discharges (above 200 Pa, 1.5 Torr) the increased ion-neutral collision frequency results in a collisionally dominated IEDF at the wall, the shape of which is largely independent of the voltage amplitude, limiting the range of accessable ion energies 12 .…”

A tutorial on radio frequency sheath physics for magnetically confined fusion devices

“…Up until now the discussion of sheaths has been limited to the physics contained in the cold-ion nonlinear fluid model NoFlu. Both static and RF sheaths have been studied with kinetic, primarily, particle-in-cell (PIC) models (Chodura 1982;Perkins 1989;Paes, Sydora & Dawson 1992;Gunn 1997;Ngadjeu et al 2011;Jenkins & Smithe 2015;Khaziev & Curreli 2015;Sharma et al 2018). Here, the additional physics provided by kinetic models is briefly summarized.…”