A molecular dynamics investigation of fluorocarbon based layer-by-layer etching of silicon and SiO2

Abstract

A molecular dynamics model is used to understand the layer-by-layer etching of Si and SiO2 using fluorocarbon and Ar+ ions. In these two-step etch processes, a nanometer-scale fluorocarbon passivation layer is grown on the material’s surface using low energy CFx+ ions or radicals. The top layers of the material are then reactive ion etched by Ar+ ions utilizing the fluorocarbon already present on the material surface. By repeating these two steps, Si or SiO2 can be etched with nanometer-scale precision and the etch rate is considerably faster than what traditional atomic layer etching techniques provide. The modeling results show that fluorocarbon passivation films can be grown in a self-limiting manner on both Si and SiO2 using low energy CF2+ and CF3+ ions. The fluorocarbon passivation layer is a few angstroms thick, and its thickness increases with the fluorocarbon ion’s energy. Increasing the ion energy, however, amorphizes the top atomic layers of the material. In addition, the fluorocarbon film becomes F rich with increasing ion energy. Simulations of fluorocarbon passivated SiO2 surface show that Ar+ ions with energy below 50eV etch Si (within SiO2) in a self-limiting manner. Si etching stops once F in the fluorocarbon passivation layer is exhausted or is pushed too deep into the substrate. Oxygen within SiO2 is more easily sputtered from the material surface than Si, and the top layers of SiO2 are expected to become O deficient during Ar+ ion bombardment. Ar+ ion etching of fluorocarbon passivated Si also appears to be self-limiting below 30eV ion energy, and etching stops once F on the material surface is either consumed or becomes inaccessible.

Atomic Layer Etching at the Tipping Point: An Overview

“…10). Similarly, the physical sputter yield data of SiO 2 for Ar + ion bombardment 95,122,124,128,[133][134][135][136][137] show significant scatter (see Fig. 11).…”

“…9 The molecular dynamics simulation of Rauf et al 95 first showed potential of a two-step etch process consisting of the formation of a nanometer-thick, self-limited fluorocarbon passivation layer on an SiO 2 or Si surface followed by etching with Ar + ions with energies up to 50 eV using the deposited fluorocarbon as a source of etchant. A sequence of these steps enabled nanometer precise etching of SiO 2 and Si.…”

“…This method is related to pulsed plasma approaches. 5,98 Using a steady-state Ar plasma in conjunction with periodic injection of a defined number of C 4 F 8 molecules and synchronized plasmabased Ar + ion bombardment, Metzler et al 99 evaluated an approach related to both simulations 9,95 and demonstrated that in agreement with the simulations Angstrom level precision in etching of SiO 2 is possible. For low energy Ar + ion bombardment conditions giving a maximum ion energy of about 20 eV, the physical sputter rate of SiO 2 vanishes.…”

“…as seen in the case of SiO 2 ALE. 9,95,99 Additionally, the somewhat relaxed expectations relative to true atomistic level control make the prospect of broad implementation more realistic.…”

“…close to the ALE win- Figure 10. Review of literature data of physical sputter rates reported for Si versus the square-root of Ar ion energy up to energies of 400 eV 95,[118][119][120][121][122][123][124][125][126][127][128][129][130][131][132] along with SRIM simulation results. The threshold for physical sputtering of Si is ≈20 eV.…”