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.

A molecular-dynamics-based model has been developed to understand etching of amorphous SiO2, with and without a fluorocarbon reactive layer, by energetic fluorocarbon (CFx+) ions. The model includes a representation of the solid and a set of interatomic potentials required for the SiO2–CFx interaction system. Two- and three-body pseudopotentials have either been obtained from published literature or computed using ab initio techniques. The Stillinger–Weber potential construct is used to represent potentials in our model and particle trajectories are advanced using the velocity-Verlet algorithm. The model is validated by comparing computed bond lengths and energies with published experimental results. Computed yield for Ar+ ion sputtering of SiO2 is also compared with published data. In the computational results described in this article, the model SiO2 test structure (with a thin fluorocarbon reactive layer) is prepared by starting with α-quartz ([001] orientation) and bombarding it with 50-eV CF2+ ions. Energetic CF2+ ions with different energies and angles of impact are then bombarded on this test structure to determine ion etch characteristics. Results show that etch yield increases with ion energy for all angles of impact. Etch yield, however, exhibits a nonlinear dependence on angle of impact with a peak around 60°. This nonlinear behavior is attributed to the balance among fraction of incident ion energy deposited in the material, ion energy deposition depth, and direction of scattering during secondary interaction events. Si in the lattice is primarily etched by F atoms and the primary Si-containing etch by-products are SiFx and SiOxFy radicals. However, oxygen either leaves the test structure as atomic O or in combination with C. While fragments of the energetic incident ion retain a substantial fraction of incident ion energy on ejection from the surface, etch by-products that have their origin in test structure atoms only have a few eV of energy on exit. Etch results are sensitive to fluorocarbon layer characteristics and etch yields decrease as the fluorocarbon reactive layer thickens.

A molecular dynamics model is used to investigate the interaction of energetic ions with fluorocarbon passivated Si, O, C, and H (SiOCH) based low-κ dielectrics. The model includes a set of interatomic potentials required for the SiOCH–CFx interaction system, where the two- and three-body pseudopotentials have either been obtained from published literature or computed using ab initio techniques. The test structure used for the ion interaction simulations is put together through deposition of low energy SiOx+, CHy+, and H+ ions on a crystalline Si substrate. A thin fluorocarbon passivation layer is grown on the low-κ test structures by bombarding them with moderate energy CFx+ ions. Simulations of CF2+ ion interaction with the fluorocarbon passivated SiOCH samples show that the sputter yield of sample constituents (Si, O, and H) increases with ion energy and peaks at about 60°. H sputters more easily compared to other species, and the surface layer is expected to become H deficient over time. Sputtered H atoms are also generated over a broader region near the surface compared to other species. Most sputtered clusters with origin in the bulk film are ejected with energies less than 10eV and their angular and energy distributions are not sensitive to the energy or angle of the incident ion. Incident CF2+ ion breaks apart on contact with the test structure and, at high energies and near normal incidence, virtually no CF2 reflects back from the sample. Fragments of the incident ion have reflectionlike properties: peak in angular dependence function shifts towards larger angles as ion angle of incidence increases and reflected fragments are more energetic as ion angle of incidence increases. Comparison of CF2+ ion etching properties of SiOCH low-κ dielectric with SiO2 shows that more atoms are sputtered from SiOCH under identical conditions. However, as many of the sputtered atoms from the SiOCH material are light H atoms, mass etch yield from SiOCH and SiO2 is comparable. Si and O are both found to sputter more easily from SiOCH relative to SiO2. SiOCH low-κ ion etching properties are compared for several ions (CFx+, CHFy+, SiFz+, and Ar+). Results show that the etch yield from F containing ions is larger than that of Ar+, and the etch yield increases as the F content of the ion increases. Comparison of the ion etching properties of porous and nonporous low-κ dielectrics shows that, under similar conditions, the fluorocarbon passivation layer is thicker on the porous material. Due to this thicker passivation layer, mass yield from the porous dielectric material is smaller for the same ion energy.

Articles you may be interested inA molecular dynamics model for the interaction of energetic ions with SiOCH low-κ dielectric

No abstract