3-dimensional modeling and simulation of surface and sidewall roughening during plasma etching

Abstract

Line edge roughness (LER) on the sidewalls of gate electrodes in metal oxide semiconductor transistors is one of the most important issues in the manufacturing of modem integrated circuits (IC). The significance of LER increases tremendously as desired features miniaturize because the dimensions of the edge roughness become comparable to those of the features. A fundamental understanding of the origins of the surface roughness and LER formation during plasma etching process is thus necessary to optimize the IC manufacturing process and prevent device failure. To meet this challenge, a 3-D Monte Carlo simulator was developed to model the roughening of a surface as well as follow the macroscopic evolution of its profile during plasma etching. The simulator employs a cellular representation of the surface with Monte Carlo modeling of the mass transport and reaction kinetics. The local geometric properties of surface features were computed by fitting to a polynomial surface, which allowed for a more accurate description of the surface normal and local curvature. This numerical algorithm for simulating etching and deposition was validated by comparing with the theoretical advancement of a surface for the case of isotropic and anisotropic etching. The simulator was used to explore surface roughening during the physical sputtering of a blanket silicon surface by argon ion bombardment. The results showed that there is a significant change in its morphology with different off-normal angles of incidence. When the surface is etched at normal incidence, the surface becomes roughened with no preferred orientation. When etched with an off-normal ion incidence below 500, the surface develops ripples that are oriented perpendicular to the ion beam direction. For off-normal angles between 500 and 600, the surface remains smooth independent of the etch time.

(cont.) The surface is again roughened for angles of incidence above 600, forming patterns that are aligned with the ion beam direction. These trends match qualitatively with those observed in experimental measurements. The simulation results further showed that these transitions of surface morphology with increasing off-normal angles of ion incidence are related to the angular dependence of the etching yield. The effects of other factors, including the angular distribution of scattered ions, the non-uniformity of the film density, the initial roughness of the surface and the re-deposition of sputtered materials were also investigated using our simulator. The roughening of the sidewalls of patterned polysilicon etched with Cl2 was also explored via simulation. The simulation results showed that the primary cause of sidewall roughness is due to the transfer of roughness on the photoresist and anti-reflective coating (ARC) sidewall to the underlying polysilicon sidewall during etching. The root-mean-square (RMS) roughness of the sidewalls due to the features that result from the etching is highest at the photoresist layer and decreases with depth. This trend is consistent with experimental observations. The simulation also showed that the re-deposition of sputtered photoresist particles onto the sidewalls during etching enhances sidewall roughening.