Transient behavior in quasi-atomic layer etching of silicon dioxide and silicon nitride in fluorocarbon plasmas

Abstract

The mechanism for atomic layer etching (ALE) typically consists of two sequential self-limited half-reactions-passivation and ion bombardment-which provide unique control over the process. Some of the possible benefits of this control include increased selectivity, reduced plasma induced damage, improved uniformity and aspect ratio independence. To achieve the greatest benefit from ALE, both half-reactions should be fully self-limited. In the experimental demonstration of ALE of SiO 2 using fluorocarbon plasmas, the passivation phase typically consists of deposition of fluoropolymer on the SiO 2 surface. This passivation step is not a self-limited reaction as the final polymer thickness depends on the passivation time. In this paper, results are presented from a computational investigation of the ALE of SiO 2 and Si 3 N 4 focusing on the implications of this nonself-limited passivation phase. The polymer overlayer was found to be critically important to the ALE performance, providing the main mechanism for selectivity between SiO 2 and Si 3 N 4. The polymer overlayer acts as a fuel for etching SiO 2 , which couples the etch depth per ALE cycle to the passivation time. Due to the inherently pulsed nature of the ALE mechanism, the polymer overlayer requires a finite number of cycles to reach a pulsed periodic steady-state thickness. Since the thickness of the polymer overlayer largely determines selectivity between SiO 2 and Si 3 N 4 , the initial formation of an overlayer results in a transient period at the beginning of etching where high selectivity may not be achieved. For the etching of thin films, or applications which require very high selectivity, this transient etching period may be a limiting factor. Results are also presented using ALE to etch high aspect ratio self-aligned contacts which could not be cleared using continuous plasma etching with similar ion energies and flux ratios.

Atomic layer etching of SiO2 with Ar and CHF3 plasmas: A self‐limiting process for aspect ratio independent etching

“…The FC layer deposition plays an opposite role. We know that the etching of SiO 2 occurs during the Ar ion bombardment phase when SiO 2 is converted to fluorinated SiO 2 by mixing with FC polymer . Since the entire FC layer must be consumed during the etch step, the amount of SiO 2 etched is proportional to the initial polymer thickness.…”

“…Since the entire FC layer must be consumed during the etch step, the amount of SiO 2 etched is proportional to the initial polymer thickness. The reaction of the FC polymer with SiO 2 happens continuously during the Ar ion bombardment step with further FC‐mixing reactions and hence SiO 2 removal happening throughout, until the polymer is fully depleted . This results in an EPC that depends on polymer thickness, which in turn depends on electrode temperature.…”

Evolution of photoresist layer structure and surface morphology under fluorocarbon‐based plasma exposure

“…Atomic fluorine is of added significance in an ~94% Ar/6% C 4 F 8 plasma admixture as used in this study, because at this composition the flux of neutral species becomes comparable to the flux of ions . Ion bombardment of the FC groups at the surface releases atomic fluorine, which begins to reduce the thickness of the DAC layer .…”

“…The etch front of the modified layer propagates deeper into the layer structure. The depth is determined by a balance between available fluorine at the greatest depth of the modified layer, which promotes additional conversion of the DAC layer into the modified layer, and ion bombardment of the surface, which etches the top of the modified layer . As the magnitude of the substrate bias increases, the modified layer etch front propagates at a faster rate because more fluorine is available from the deposited surface FC groups and can be driven more deeply into the underlayer .…”

“…The reduction in the DAC layer thickness has a corresponding impact on the bulk layer thickness. The main difference between the oxygen and C 4 F 8 gases in impacting the layer structure is that the rate of modification from C 4 F 8 is slower than that from oxygen, because the FC deposited on the surface functions as an additional passivating layer that mediates the existing etching interaction between the DAC layer and underlying bulk layer …”