Electric field reversals resulting from voltage waveform tailoring in Ar/O₂ capacitively coupled plasmas sustained in asymmetric systems

Abstract

The etching of nanometer scale high-aspect-ratio (HAR) features into dielectric materials in low pressure radio frequency excited plasmas is often accompanied by charge accumulation inside the features which can slow etching rates and produce distortions such as twisting. The intra-feature charging is at least partially produced by differences in electron and ion energy and angular distributions (EADs). Positive ions, accelerated to high energies having narrow angular spreads by the sheath electric field, can penetrate deeply into HAR features. Electrons typically arrive at the wafer with nearly thermal and isotropic distributions and do not penetrate deeply into HAR features. These disparities lead to differential charging of the inside of the feature, which can lead to reductions in etch rate and feature distortion due to ion deflection. With increasing aspect ratio of features, charging challenges are expected to continue for the foreseeable future. In this work, the use of tailored voltage waveforms in geometrically asymmetric capacitively coupled plasmas sustained in Ar/O 2 at 40 mTorr was computationally investigated with the goal of shaping the EAD of electrons incident onto the substrate to address differential charging. The tailored waveform consisted of a sinusoidal wave and its higher harmonics with a fundamental frequency of 1 MHz. We found that electric field reversals (EFRs) in the sheath and presheath can occur during the anodic portion of the cycle. The EFR increases the energy and decreases the angular spread of electrons incident onto the substrate. The magnitude of the EFR can be controlled by the phase angle of the even harmonics and the gas composition. Due to its electronegative nature, increasing mole fractions of O 2 impedes electron transport to the surface which further increases the EFR.

Control of ion flux-energy distributions by low frequency square-shaped tailored voltage waveforms in capacitively coupled plasmas

“…In this case, however, the variation of the relative phases between the harmonic components provides additional control over the particular shape of the applied voltage waveform, realizing a variant of 'voltage waveform tailoring' (VWT). An interesting consequence of this method is the introduction of the 'electrical asymmetry effect' [15,16] resulting in the controllable development of a significant DC self-bias voltage even in the case of geometrically symmetric systems, as it has been shown in a series of numerical studies [17][18][19][20][21][22][23][24][25]. In principle, by applying a high enough number of harmonics, any arbitrary shape of the voltage waveform can be achieved, although technical limitations (frequency bandwidth, impedance matching, etc) do restrict the feasible harmonic number [26][27][28].…”

High energy electrons induced by nonlinear effect in synchronized dual-level radio frequency pulsing capacitively coupled plasmas

“…Both the high energy electrons accelerated by the reversed electric field filaments and by the opposite sheath expansion will play an important role in migrating the surface charging effects during etching insulator. In the case of CW, the nonlinear electric field is negligible, however, the high-energy electrons still exist (∼25 eV), which are mainly from the opposite sheath acceleration and the electric field reversals during sheath collapse, and can also be observed in figures 5(a5) and (b5) [10,25].…”

“…Many promising technique candidates for migrating the charging effects, include DC capacitively coupled plasma (CCP) [8,9], tailored voltage waveforms [6,10], and pulse modulated radio frequency (RF) discharge [11,12]. The first method is to introduce an additional negative DC source into traditional CCP, which produces sufficiently high energy secondary electrons to overcome the sheath potential barrier and reach the trench bottom, neutralizing the positive surface charges [8].…”

Science-based, data-driven developments in plasma processing for material synthesis and device-integration technologies

“…[129][130][131][132] Electric field reversals (EFRs) in the sheath and presheath, the electronegative nature, and increasing mole fractions of O 2 impede electron transport to the surface, which further increases EFR. [133][134][135] The 2D axisymmetric plasma sheath model predicted ion trajectory deviations at the plasma-wafer interface for actual chamber geometries and etch conditions. 136) The plasma sheath was deformed, and the ion trajectories tilted from the normal to the surface in the vicinity of the wafer edge.…”