Emilie Despiau-Pujo

Elementary processes of H2 plasma-graphene interaction: A combined molecular dynamics and density functional theory study

Elementary interactions between H atoms and monolayer graphene are investigated using classical molecular dynamics (CMD) and density functional theory (DFT). C-H interatomic potential curves and associated energy barriers are reported depending on the H impact position (top, bridge, hollow, vacancy, or edge sites of graphene nanoribbons). Chemisorption of atomic hydrogen and formation of molecular hydrogen from chemisorbed H states on graphene are examined. The influence of graphene temperature and incident species energy on adsorption, reflection, and penetration mechanisms is also presented. Except for impacts at graphene nanoribbon (GNR) edges or at defect locations, H atoms are shown to experience a repulsive force due to delocalized π-electrons which prevents any species with less than 0.4-0.6 eV to chemisorb on the graphene surface. C-H bond formation requires a local sp2-sp3 rehybridization resulting in structural changes of the graphene sample. Chemisorption sites with deep potential wells and no ...

MD simulations of low energy Clx+ ions interaction with ultrathin silicon layers for advanced etch processes

Molecular dynamics simulations of low-energy (5–100 eV) Cl+ and Cl2+ bombardment on (100) Si surfaces are performed to investigate the impact of plasma dissociation and very low-energy ions (5–10 eV) in chlorine pulsed plasmas used for silicon etch applications. Ion bombardment leads to an initial rapid chlorination of the Si surface followed by the formation of a stable SiClx mixed layer and a constant etch yield at steady state. The SiClx layer thickness increases with ion energy (from 0.7 ± 0.2 nm at 5 eV to 4 ± 0.5 nm at 100 eV) but decreases for Cl2+ bombardment (compared to Cl+), due to the fragmentation of Cl2+ molecular ions into atomic Cl species with reduced energies [one X eV Cl +  two 2X eV Cl2+]. The Si etch yield is larger for Cl2+ than Cl+ bombardment at high-energy (Ei > 25 eV) but larger for Cl+ than Cl2+ bombardment at low-energy (Ei  two 2X eV Cl2+]. The Si etch yield is larger for Cl2+ than Cl+ bombardment at high-energy (Ei > 25 eV) but larger for Cl+ than Cl2+ bombardment at low-energy (Ei < 25 eV) due to threshold effects. And the higher the ion energy, the less saturated the etch products. Results suggest that weakly dissociated chlorine ...

MD simulations of GaN sputtering by Ar+ ions: Ion-induced damage and near-surface modification under continuous bombardment

Results from molecular dynamics simulations of continuous 50–200 eV Ar+ bombardment on wurtzite and zinc blende GaN surfaces are reported. A new analytical bond-order potential, originally developed for growth process studies, is used to investigate the low-energy physical sputtering of GaN compounds. Preferential sputtering of N atoms is initially observed up to 3.5×1015 ions/cm2 fluence, after which the layers reach steady state sputtering. The crystalline structure of the GaN sample does not have a major influence on the sputtering yield due to the rapid amorphization of the top surface after a few hundred impacts. Concentration depth profiles indicate a surface enrichment in gallium with a N/Ga concentration ratio equal to 0.59±0.1 for 100 eV bombardment, in agreement with published experimental studies. For the same conditions, Ga, N, and GaN species represent 25, 60, and 7% of the sputtered products. A significant fraction of those products leave the surface with kinetic energies sufficiently high t...

Etching mechanisms of graphene nanoribbons in downstream H2 plasmas: insights from molecular dynamics simulations

Lateral etching mechanisms of graphene nanoribbons (GNRs) with zigzag (ZZ) edges in downstream H2 plasmas are investigated using molecular dynamics simulations. A new etching mechanism is found, which occurs in three consecutive phases and requires a continuous exposure of GNRs to H atoms and high substrate temperatures (~800 K). Full hydrogenation of GNR free edges during phase 1 reduces the potential barriers to H chemisorption on near-edge C atoms from the basal plane. Subsequent hydrogenation of near-edge C–C dimers creates mechanical stress between C atoms (due to local sp2-to-sp3 rehybridizations) which leads to the rupture of C–C dimers bonds, unzipping locally the 1st and 2nd edge carbon rows. The unzipping then propagates randomly along the GNR edges and creates suspended linear carbon chains (phase 2). Weakened by their exposure to continuous H bombardment and strong thermal vibrations, the suspended carbon chains may then rupture, leading to the sputtering of their carbon atoms as single C atoms or C2 molecules (phase 3). Thus no formation of volatile hydrocarbon etching products is observed in this three-phase mechanism, which explains why the ribbon edges can be sharp-cut without generation of line-edge roughness, as also observed experimentally. Influence of substrate temperature on ZZ-GNRs etching is investigated and suggests the dominant mechanisms for understanding the temperature dependence of the etch rate observed experimentally (peaks at 800 K and decreases for lower or higher temperatures).

Molecular dynamics simulations of GaAs sputtering under low-energy argon ion bombardment

Results from molecular dynamics (MD) simulations of low-energy (50–200eV) Ar+ ion bombardment on (110) GaAs surfaces are reported. A new analytical bond-order potential, originally developed for molecular beam epitaxy studies, is used and tested in the context of etching to investigate the nature and effects of physical sputtering on GaAs compounds. It is found that a thermal desorption model, which accounts for long time scale phenomena between MD simulated impacts, is necessary to achieve steady state sputtering. An initial rapid etch of both atomic species is observed up to 4×1016ions∕cm2 fluence with preferential sputtering of Ga atoms. At high fluences, simulations show the formation of an As-rich layer on the top surface, a subsurface enrichment of Ga, and a return to stoichiometry deeper in the solid. More than 97% of sputtered or desorbed species appear to be Ga or As atoms; sputtering of GaAs molecules is negligible. All these observations are in agreement with published experimental results. Fin...

H+ ion-induced damage and etching of multilayer graphene in H2 plasmas

H+ ion-induced damage of multilayer graphene (MLG) is investigated using Molecular Dynamics simulations as H2 plasmas could provide a possible route to pattern graphene. Low-energy (5–25 eV) H+ cumulative bombardment of ABA-stacked MLG samples shows an increase of the hydrogenation rate with the ion dose and ion energy. At 5 eV, the H coverage grows with the ion fluence only on the upper-side of the top layer but saturates around 35%. Hydrogenation of multi-layers and carbon etching are observed at higher energies. Layer-by-layer peeling/erosion of the MLG sample is observed at 10 eV and occurs in two phases: the MLG sample is first hydrogenated before carbon etching starts via the formation of CHx (∼60%) and C2Hx (∼30%) by-products. A steady state is reached after an ion dose of ∼5 × 1016 H+/cm2, as evidenced by a constant C etch yield (∼0.02 C/ion) and the saturation of the hydrogenation rate. At 25 eV, an original etching mechanism—lifting-off the entire top layer—is observed at low fluences due to the...

Key plasma parameters for nanometric precision etching of Si films in chlorine discharges

Ultrathin layered films in new transistors architectures (FinFET and fully depleted SOI) require damage-free plasma etching techniques with unprecedented selectivity between materials. To assist the development of advanced processes, molecular dynamics simulations are performed to quantify modifications (plasma-induced damage, etch rate) of Si films after exposition to various Cl2 plasma conditions, simulated by bombarding the substrate with both ion (Cl+, Cl2+) and neutral (Cl, Cl2) species. All simulations show the formation of a stable SiClx reactive layer and a constant etch yield at steady state. The key plasma parameter to control the etching of ultrathin Si layers is the ion energy (Ei), which lowers significantly both the damaged layer thickness (from 1.8 nm at 100 eV to 0.8 nm at 5 eV when Γ = 100) and the etch yield when it is decreased. The neutral-to-ion flux ratio (Γ) is the second key parameter: its increase reduces the damaged layer thickness (from 1.8 nm for Γ = 100 to 1.1 nm for Γ = 1000 ...

Pulsed Cl2/Ar inductively coupled plasma processing: 0D model versus experiments

Comparisons between measurements and spatially-averaged (0D) simulations of low-pressure Ar and Cl2 pulsed-plasmas in an industrial inductively coupled reactor are reported. Our analysis focuses on the impact of the pulsing parameters (frequency f, duty cycle dc) on the chemical reactivity of the plasma and on the ion fluxes to the walls. Charged particle densities and ion fluxes are highly modulated when the plasma is pulsed at 1 kHz 1 kHz. However, the dc strongly influences the Cl2/Cl density ratio and is an excellent knob for controlling the plasma chemical reactivity: the higher the dc the higher the Cl density. The trends and quantities in the 0D simulation are in close agreement with experiments. This proves the capacity of global models to reproduce the fundamental features of pulsed plasmas in simple chemistries and to assist the development of pulsed processes.

Simulations of radical and ion fluxes on a wafer in a Cl2/Ar inductively coupled plasma discharge: Confrontation with GaAs and GaN etch experiments

A two-dimensional fluid model is used to study an industrial Ar/Cl2 inductively coupled plasma discharge designed to etch III-V samples. The effect of rf power, gas pressure, and chlorine content on the fluxes of reactive species reaching the wafer is numerically investigated. To understand how the etch process is influenced by the discharge conditions, simulation results are confronted with GaAs and GaN etch experiments performed in the same reactor geometry. When the source power is increased, the measured etch rate increase is consistent with the Cl radical and ion fluxes increase shown in the simulation, as well as the ion energy decrease due to the constant value of the wafer-holder power. Increasing the gas pressure results in a moderate increase in the etch rate due to the lower magnitude, lower mean energy, and anisotropy of the ion flux at high pressure. When the chlorine content is increased, the total ion flux decreases while Cl and Cl2 neutral fluxes increase significantly. A good correlation is obtained between calculated fluxes and etch characteristics, analyzed with scanning electron microscope images of etch profiles.A two-dimensional fluid model is used to study an industrial Ar/Cl2 inductively coupled plasma discharge designed to etch III-V samples. The effect of rf power, gas pressure, and chlorine content on the fluxes of reactive species reaching the wafer is numerically investigated. To understand how the etch process is influenced by the discharge conditions, simulation results are confronted with GaAs and GaN etch experiments performed in the same reactor geometry. When the source power is increased, the measured etch rate increase is consistent with the Cl radical and ion fluxes increase shown in the simulation, as well as the ion energy decrease due to the constant value of the wafer-holder power. Increasing the gas pressure results in a moderate increase in the etch rate due to the lower magnitude, lower mean energy, and anisotropy of the ion flux at high pressure. When the chlorine content is increased, the total ion flux decreases while Cl and Cl2 neutral fluxes increase significantly. A good correlation ...

Cleaning graphene: A first quantum/classical molecular dynamics approach

Graphene outstanding properties created a huge interest in the condensed matter community and unprecedented fundings at the international scale in the hope of application developments. Recently, there have been several reports of incomplete removal of the polymer resists used to transfer as-grown graphene from one substrate to another, resulting in altered graphene transport properties. Finding a large-scale solution to clean graphene from adsorbed residues is highly desirable and one promising possibility would be to use hydrogen plasmas. In this spirit, we couple here quantum and classical molecular dynamics simulations to explore the kinetic energy ranges required by atomic hydrogen to selectively etch a simple residue—a CH3 group—without irreversibly damaging the graphene. For incident energies in the 2–15 eV range, the CH3 radical can be etched by forming a volatile CH4 compound which leaves the surface, either in the CH4 form or breaking into CH3 + H fragments, without further defect formation. At t...