Nobuya Nakazaki

Molecular dynamics simulations of silicon chloride ion incidence during Si etching in Cl-based plasmas

Classical molecular dynamics (MD) simulations have been performed for SiClx+ (x = 0–4) ions incident on Si(100) surfaces, using an improved Stillinger–Weber (SW) potential form, to understand the surface reaction kinetics of etch byproduct ion incidence during Si etching in Cl-based plasmas. The ions were normally incident on surfaces with translational energies in the range of Ei = 20–500 eV, and the surface reaction kinetics of Clx+ (x = 1, 2) ion incidence were also simulated for reference. The etch yields and thresholds presently simulated were in agreement with the experimental results previously reported for the respective ion beam incidences on Si. Numerical results indicated that the etch yields y* per halogen (or per constituent Cl atom of incident ions), thresholds, surface coverages of Cl atoms adsorbed, and thicknesses of chlorinated surface layers are almost the same, when compared at the same translational energy per halogen; moreover, the stoichiometries of product species desorbed, stoichiometries of chlorinated surface layers, and their depth profiles are also similar when compared at the same . Thus, it follows that the etching characteristics for SiClx+ as well as Clx+ incidences on Si are determined primarily or scaled universally by , unless the deposition is significant at low Ei or for SiCl+ and SiCl2+.

Surface roughening and rippling during plasma etching of silicon: numerical investigations and a comparison with experiments

Atomic- or nanometer-scale surface roughening and rippling during Si etching in high-density Cl2 and Cl2/O2 plasmas have been investigated by developing a three-dimensional atomic-scale cellular model (ASCeM-3D), which is a 3D Monte Carlo-based simulation model for plasma–surface interactions and the feature profile evolution during plasma etching. The model took into account the behavior of Cl+ ions, Cl and O neutrals, and etch products and byproducts of SiClx and SiClxOy in microstructures and on feature surfaces therein. The surface chemistry and kinetics included surface chlorination, chemical etching, ion-enhanced etching, sputtering, surface oxidation, redeposition of etch products desorbed from feature surfaces being etched, and deposition of etch byproducts coming from the plasma. The model also took into account the ion reflection or scattering from feature surfaces on incidence and/or the ion penetration into substrates, along with geometrical shadowing of the feature and surface reemission of n...

Molecular dynamics simulations of Si etching in Cl- and Br-based plasmas : Cl+ and Br+ ion incidence in the presence of Cl and Br neutrals

Classical molecular dynamics (MD) simulations have been performed for Cl+ and Br+ ions incident on Si(100) surfaces with Cl and Br neutrals, respectively, to gain a better understanding of the ion-enhanced surface reaction kinetics during Si etching in Cl- and Br-based plasmas. The ions were incident normally on surfaces with translational energies in the range Ei = 20–500 eV, and low-energy neutrals of En = 0.01 eV were also incident normally thereon with the neutral-to-ion flux ratio in the range Γn0/Γi0 = 0–100, where an improved Stillinger--Weber potential form was employed for the interatomic potential concerned. The etch yields and thresholds presently simulated were in agreement with the experimental results previously reported for Si etching in Cl2 and Br2 plasmas as well as in Cl+, Cl2+, and Br+ beams, and the product stoichiometry simulated was consistent with that observed during Ar+ beam incidence on Si in Cl2. Moreover, the surface coverage of halogen atoms, halogenated layer thickness, surface stoichiometry, and depth profile of surface products simulated for Γn0/Γi0 = 100 were in excellent agreement with the observations depending on Ei reported for Si etching in Cl2 plasmas. The MD also indicated that the yield, coverage, and surface layer thickness are smaller in Si/Br than in Si/Cl system, while the percentage of higher halogenated species in product and surface stoichiometries is larger in Si/Br. The MD further indicated that in both systems, the translational energy distributions of products and halogen adsorbates desorbed from surfaces are approximated by two Maxwellians of temperature T1 ≈ 2500 K and T2 ≈ 7000–40 000 K. These energy distributions are discussed in terms of the desorption or evaporation from hot spots formed through chemically enhanced physical sputtering and physically enhanced chemical sputtering, which have so far been speculated to both occur in the ion-enhanced surface reaction kinetics of plasma etching.

Two modes of surface roughening during plasma etching of silicon: Role of ionized etch products

Atomic- or nanometer-scale surface roughening has been investigated during Si etching in inductively coupled Cl2 plasmas, as a function of rf bias power or ion incident energy Ei, by varying feed gas flow rate, wafer stage temperature, and etching time. The experiments revealed two modes of surface roughening which occur depending on Ei: one is the roughening mode at low Ei < 200–300 eV, where the root-mean-square (rms) roughness of etched surfaces increases with increasing Ei, exhibiting an almost linear increase with time during etching (t < 20 min). The other is the smoothing mode at higher Ei, where the rms surface roughness decreases substantially with Ei down to a low level < 0.4 nm, exhibiting a quasi-steady state after some increase at the initial stage (t < 1 min). Correspondingly, two different behaviors depending on Ei were also observed in the etch rate versus Ei curve, and in the evolution of the power spectral density distribution of surfaces. Such changes from the roughening to smoothing mo...

Surface smoothing during plasma etching of Si in Cl2

Effects of initial roughness on the evolution of plasma-induced surface roughness have been investigated during Si etching in inductively coupled Cl2 plasmas, as a function of rf bias power or ion incident energy in the range Ei ≈ 20–500 eV. Experiments showed that smoothing of initially rough surfaces as well as non-roughening of initially planar surfaces can be achieved by plasma etching in the smoothing mode (at high Ei) with some threshold for the initial roughness, above which laterally extended crater-like features were observed to evolve during smoothing. Monte Carlo simulations of the surface feature evolution indicated that the smoothing/non-roughening is attributed primarily to reduced effects of the ion scattering or reflection from microscopically roughened feature surfaces on incidence.

Ripple formation on Si surfaces during plasma etching in Cl2

Nanoscale surface roughening and ripple formation in response to ion incidence angle has been investigated during inductively coupled plasma etching of Si in Cl2, using sheath control plates to achieve the off-normal ion incidence on blank substrate surfaces. The sheath control plate consisted of an array of inclined trenches, being set into place on the rf-biased electrode, where their widths and depths were chosen in such a way that the sheath edge was pushed out of the trenches. The distortion of potential distributions and the consequent deflection of ion trajectories above and in the trenches were then analyzed based on electrostatic particle-in-cell simulations of the plasma sheath, to evaluate the angular distributions of ion fluxes incident on substrates pasted on sidewalls and/or at the bottom of the trenches. Experiments showed well-defined periodic sawtooth-like ripples with their wave vector oriented parallel to the direction of ion incidence at intermediate off-normal angles, while relatively weak corrugations or ripplelike structures with the wave vector perpendicular to it at high off-normal angles. Possible mechanisms for the formation of surface ripples during plasma etching are discussed with the help of Monte Carlo simulations of plasma-surface interactions and feature profile evolution. The results indicate the possibility of providing an alternative to ion beam sputtering for self-organized formation of ordered surface nanostructures.Nanoscale surface roughening and ripple formation in response to ion incidence angle has been investigated during inductively coupled plasma etching of Si in Cl2, using sheath control plates to achieve the off-normal ion incidence on blank substrate surfaces. The sheath control plate consisted of an array of inclined trenches, being set into place on the rf-biased electrode, where their widths and depths were chosen in such a way that the sheath edge was pushed out of the trenches. The distortion of potential distributions and the consequent deflection of ion trajectories above and in the trenches were then analyzed based on electrostatic particle-in-cell simulations of the plasma sheath, to evaluate the angular distributions of ion fluxes incident on substrates pasted on sidewalls and/or at the bottom of the trenches. Experiments showed well-defined periodic sawtooth-like ripples with their wave vector oriented parallel to the direction of ion incidence at intermediate off-normal angles, while relatively...