Hirotaka Tsuda

Atomic-Scale Cellular Model and Profile Simulation of Si Etching: Analysis of Profile Anomalies and Microscopic Uniformity

Reactive ion etching (RIE) has been used in the manufacture of semiconductor integrated circuit devices. However, the formation mechanisms of profile anomalies and microscopic uniformity have been poorly understood until now. In this paper, we focus on the analysis of formation mechanisms of profile anomalies and microscopic uniformity during plasma etching of Si in Cl2 plasmas, using our own atomic-scale cellular model (ASCeM). The numerical results indicated that high neutral-to-ion flux ratios result in microtrench formation. Moreover, RIE lag tends to occur at low neutral-to-ion flux ratios (<50), whereas inverse RIE lag occurs at high neutral-to-ion flux ratios in typical low-pressure and high-density plasmas. In particular, the etch rates for narrow patterns (<70 nm) increase significantly with increasing neutral-to-ion flux ratio. The synergistic effects between ion-enhanced etching and neutral shadowing in microstructural features play a significant role in the formation of profile anomalies.

Modeling and simulation of nanoscale surface rippling during plasma etching of Si under oblique ion incidence

A three-dimensional atomic-scale cellular model (ASCeM-3D) has been developed to reproduce the evolution of feature profiles on atomic or nanometer scale during plasma etching. Emphasis was placed on the evolution of nanoscale surface features and roughness during Si etching in Cl2 plasmas, with further attention being given to that of ripple structures on etched surfaces. Simulations were carried out for different ion incident angles from θi = 0 to 85°, with an ion incident energy Ei = 100 eV, flux Γi0 = 1.0 ×1016 cm-2 s-1, and neutral-to-ion flux ratio Γn0/Γi0 = 100, which are typical in high-density plasma etching environments. Numerical results indicated that as the angle θi is increased, nanoscale concavo-convex features drastically change and ripple structures occur on etched surfaces. For θi = 0° or normal ion incidence, the surfaces are randomly roughened. For increased θi = 30–45° or oblique ion incidence, the ripples are formed perpendicular to the direction of ion incidence, while they are parallel to the direction of ion incidence for further increased θi = 75–80°. Analysis of ion trajectories implied that the ion reflection and concentration on microscopically roughened surfaces largely affect the surface roughening and rippling during plasma etching. These numerical approaches would become important to predict the nanoscale surface features and roughness, especially the line edge roughness (LER) formed on feature sidewalls, because experiments of oblique incidence of ions on surfaces are difficult in plasma environments.

Molecular-Dynamics-Based Profile Evolution Simulation for Sub-10-nm Si Processing Technology

Fully atomistic profile evolution simulations in dry etching processes have been demonstrated by classical molecular dynamics (MD) simulations. In our first attempt, this technique was applied to 5 nm Si trench etching by halogen (F, Cl, and Br) mono-energetic beams. The trench profile evolutions were dynamically reproduced on the atomic scale and qualitatively agreed with our common view based on the chemical properties of the injected species. This MD-based ab-initio approach is not only a baseline to verify the existing continuum-model-based simulation but a tool to estimate electrical performance including the effects of damaging layers and process margins/yields in the sub-10-nm processing rules.

Metrology and inspection required for next generation lithography

We summarize the metrology and inspection required for the development of nanoimprint lithography (NIL) and directed self-assembly (DSA), which are recognized as candidates for next generation lithography. For NIL, template inspection and residual layer thickness (RLT) metrology are discussed. An optical-based inspection tool for replica template inspection showed sensitivity for defects below 10 nm with sufficient throughput. Scatterometry was applied for RLT metrology. Feedback control with scatterometry improved RLT uniformity across an imprinting field. For DSA, metrology for image placement and cross-sectional profile are addressed. Design-based scanning electron microscope (SEM) metrology utilizing a die-to-database electron beam (EB) inspection tool was effective for image placement metrology. For the cross-sectional profile, a holistic approach combining scatterometry and critical dimension SEM was developed. The technologies discussed here will be important when NIL and DSA are applied for IC manufacturing, as well as in the development phases of those lithography technologies.

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.

Three-Dimensional Atomic-Scale Cellular Model and Feature Profile Evolution during Si Etching in Chlorine-Based Plasmas: Analysis of Profile Anomalies and Surface Roughness

Three-dimensional measurement and prediction of atomic-scale surface roughness on etched features become increasingly important for the fabrication of next-generation devices; however, the feature profiles are too small or too complex to measure the surface roughness on bottom surfaces and sidewalls of the etched features. To predict the surface roughness on an atomic or nanometer scale, we developed our own three-dimensional atomic-scale cellular model (ASCeM-3D) and feature profile simulation, with emphasis being placed on the formation of surface roughness on the atomic scale soon after the start of Si etching in Cl2 plasmas. Numerical results indicated that nanometer-scale convex roughened features appear on the surface soon after the start of etching, which causes the formation of a larger surface roughness, and that the surface roughness tends to be saturated after several seconds. In effect, the nanoscale convex features increase in size with increasing etching or plasma exposure time, and new nanoscale convex ones continue to appear on top of the enlarged convex ones during etching, thus resulting in concavo-convex features superimposed on the roughened surface. A comparison was also made between numerical results and experiments.

Molecular Dynamics Analysis of the Formation of Surface Roughness during Si Etching in Chlorine-Based Plasmas

Addition of oxygen to Cl2 discharge is widely used in Si etching for the fabrication of gate electrodes and shallow trench isolation. As the control of etching processes becomes more critical, a deeper understanding of plasma-surface interactions is required for the formation of roughened surfaces during etching. In particular, a small amount of O2 often leads to profile anomalies such as residues, micropillars, and roughened surfaces. In this study, we focus on the mechanism underlying local surface oxidation during Si etching in Cl2/O2 plasmas, and analyze the relationship between local surface oxidation and surface roughness on the nanometer scale, by a classical molecular dynamics (MD) simulation. The numerical results indicated that O radicals tend to break Si–Si bonds and distort the Si lattice structure; thus, nanometer-scale micromasks tend to be formed on convex roughened surfaces, owing to the reactivity of O radicals with substrate Si atoms and Cl atoms. The results also imply that the nanometer-scale micromasks significantly affect the formation of roughened surfaces and evolution of micropillars.

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...