Low-energy argon ion bombardment-induced decomposition of physisorbed hydrofluorocarbons on silicon nitride surfaces: A computational mechanistic study

Abstract

Using a combination of tight binding molecular dynamics and ab initio molecular dynamics simulations, we study the mechanisms of bombardment-induced decomposition of hydrofluorocarbons (HFCs) physisorbed on silicon nitride for ion energies of ≤35 eV. We propose three key mechanisms by which bombardment-driven HFC decomposition can occur, focusing on the two pathways observed at these low ion energies: “direct decomposition” and “collision assisted surface reactions (CASRs).” Our simulation results clearly demonstrate the importance of the presence of favorable reaction coordinates for enabling CASR, which dominates at lower energies (≈11 eV). At higher energies, direct decomposition becomes more favored. Our work also predicts that the primary decomposition pathways for CH3F and CF4 are CH3F → CH3 + F and CF4 → CF2 + 2F, respectively. The fundamental details of these decomposition pathways and the decomposition products formed under ion bombardment have implications for plasma-enhanced atomic layer etching process design that will be discussed.

Silicon Nitride Surface Modification by Ion Bombardment with Physisorbed Hydrofluorocarbons: Theoretical Assessment

“…At 35 eV, both windows are active; the ion energy is sufficiently high to both induce the initial surface reaction and drive atoms from newly formed surface moieties into the film. We have also shown lower energies to be sufficient for driving surface reactivity . However, simple physics would suggest that those lower energies are not as capable of implanting ions to the same depth.…”

“…These sites may include lone pairs, as well as transiently created surface defects, as shown in Figure . In the depicted example, the 35 eV bombardment-induced cleavage of the C–F bond via a direct decomposition mechanism (as discussed in our previous work) initiates the process, releasing a free F anion. Though this mechanism of decomposition has already been discussed in our previous work, here, we focus on the role of the surface in the pathway.…”

“…As discussed in previous work, reactions between physisorbed HFCs and the SiN surface are enabled/enhanced by ion bombardment through two key mechanisms: direct decomposition and collision-assisted surface reactions (CASR). In these initial instances of surface modification, reactive fragments resulting from these decomposition processes can react with sites near the surface.…”

“…We note that the starting model slab used in this work is the same that was used in a previously published work. 20 ■ RESULTS AND DISCUSSION Mechanisms of HFC Modification. Using our novel direct simulation approach, we have simulated the structural and compositional evolution of SiN substrates with physisorbed HFC monolayers (here, we focus on CF 4 and CH 3 F) at 35 eV, normally incident Ar bombardment.…”

“…Also note the significantly greater amounts of F produced from CF 4 decomposition, as studied in previous work. 20 Bonding and Stoichiometry of Modified Film. As expected, F and C are readily incorporated into the film when a physisorbed monolayer of HFCs is bombarded.…”