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

Abstract

Surface loss rates of H and Cl radicals in an inductively coupled plasma etcher derived from time-resolved electron density and optical emission measurementsModeling of fluorine-based high-density plasma etching of anisotropic silicon trenches with oxygen sidewall passivation Atomic-or nanometer-scale surface roughening and rippling during Si etching in high-density Cl 2 and Cl 2 /O 2 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 þ ions, Cl and O neutrals, and etch products and byproducts of SiCl x and SiCl x O y 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 neutrals. The simulation domain was taken to consist of small cubic cells of atomic size, and the evolving interfaces were represented by removing Si atoms from and/or allocating them to the cells concerned. Calculations were performed for square substrates 50 nm on a side by varying the ion incidence angle onto substrate surfaces, typically with an incoming ion energy, ion flux, and neutral reactant-to-ion flux ratio of E i ¼ 100 eV, C i 0 ¼ 1.0 Â 10 16 cm À2 s À1 , and C n 0 /C i 0 ¼ 100. Numerical results showed that nanoscale roughened surface features evolve with time during etching, depending markedly on ion incidence angle; in effect, at h i ¼ 0 or normal incidence, concavo-convex features are formed randomly on surfaces. On the other hand, at increased h i ¼ 45 or oblique incidence, ripple structures with a wavelength of the order of 15 nm are formed on surfaces perpendicularly to the direction of ion incidence; in contrast, at further increased h i ! 75 or grazing incidence, small ripples or slitlike grooves with a wavelength of <5 nm are formed on surfaces parallel to the direction of ion incidence. Such surface roughening and rippling in response to ion incidence angle were also found to depend significantly on ion energy and incoming fluxes of neutral reactants, oxygen, and etch byproducts. Two-dimensional power spectral density analysis of the roughened feature surfaces simulated was employed in some cases to further characterize the lateral as well as vertical extent of the roughness. The authors discuss possible mechanisms responsible for the formation and evolution of the surface roughness and ripples during plasma etching, including stochastic roughening, local micromasking, and effects of ion reflection, surface temp...

Origin of plasma-induced surface roughening and ripple formation during plasma etching: The crucial role of ion reflection

“…In simulations, as also shown in Fig. 2 partly for θ i = 45°, the rms roughness for r i > r i * exhibits a gradual increase with time to reach quasi-steady state in t < 120 s at θ i = 0°and 45°, while it exhibits a continuous increase during etching at θ i = 75°; 7,72,73…”

“…The ASCeM-3D is concerned with plasma-surface interactions and feature profile evolution during Si etching in Cl-based plasmas, which has been detailed previously. 7,72,73 Briefly, the model takes into account the transport and surface reaction kinetics of Cl + ions, Cl and O neutrals, and etch/sputter products and byproducts of SiCl x and SiCl x O y in microstructural features. The simulation domain consists of small cubic cells of atomic size (L = ρ s −1/3 = 2.7 Å with ρ s = 5.0 × 10 22 cm −3 , the atomic density of Si substrates), where the evolving interfaces are represented by removing Si atoms from and/or allocating them to the substrate surface cells concerned.…”

“…where Y is the total number of Si atoms desorbed from surfaces per ion impact, A is a proportional constant or scaling factor (taken to rely on surface conditions 7,72,73 ), and E th is the threshold energy concerned. In plasma etching where the ion-enhanced etching reactions dominate over sputtering, the etch yield is assumed to be maximum at normal θ = 0°, while the sputter yield peaks at off-normal θ ≈ 60°-70°.…”

“…[54][55][56] We have investigated surface roughening and ripple formation during Si etching in Cl-based plasmas: 7,72-78 our MC-based three-dimensional atomic-scale cellular model (ASCeM-3D), mechanistically including the effects of ion reflection, simulated the roughening and rippling in response to ion incidence angle θ i . 72,73 Moreover, the experiments of inductively coupled plasma (ICP) etching of Si in Cl 2 with different ion incident energies of E i ≈ 20−500 eV (θ i = 0°) exhibited two modes of surface rougheing: 74,75 roughening mode at low E i (< 200−300 eV) and smoothing or non-roughening mode at higher E i . A comparison of the experiments and MC simulations with the help of MD 76,77 and plasma diagnostics 74 suggested that the roughening-smoothing transition observed is characterized by reduced effects of the ion reflection, caused by a change in the predominant ion flux due to that in plasma conditions.…”

Advanced simulation technology for etching process design for CMOS device applications

“…Sawin and coworkers [48][49][50][51][52] proposed 2D and 3D cell removable models, which can reproduce the etch yields, micro-trench structures, and surface compositions for the etching of Si and SiO 2 films using Cl 2 , HBr, and C 4 F 8 =Ar plasmas. Tsuda and coworkers 53,54) have developed a model on the basis of the cell removable model with atomic-level interactions between the incident particles and the surface Si atoms in the 3D space. They demonstrated surface roughening and rippling during Si etching by Cl plasmas by considering the surface reactions in detail and proposed a mechanism for these reactions that take into account the variations in ion incident angle and gas flux ratio.…”