Study of plasma-surface kinetics and simulation of feature profile evolution in chlorine etching of patterened polysilicon

This work characterized the chlorine ion-enhanced plasma-surface interactions with polysilicon, silicon dioxide, and photoresist as a function of the plasma compositions (Cl , Cl, Cl2, SiCl2 ,), ion energy (Eion ), ion incident angle (φion ) and other important process parameters. A direct beam-scattering apparatus capable of generating low energy ions (20-100 eV) and reactive neutral species and controlling their flux levels independently was utilized to simulate accurately a high-density plasma environment. Etching of polysilicon, silicon dioxide, and photoresist was quantified to better describe etching of patterned polysilicon in fabricating the gate electrode of a transistor in the IC manufacturing process. These kinetic measurements facilitate the formulation of surface kinetic models usable in a Monte Carlo-based profile simulator, and enable predictive simulations of feature profiles evolutions. The ion-enhanced etching yields of polysilicon, silicon dioxide and photoresist were characterized as a function of ion energy, ion flux, neutral/ion flux ratio, the flux of etching by-products, and angle of ion empingement. The etching yields of all three materials by Ar and Cl increased with the increase of flux ratios and gradually saturated at high flux ratios as the depletion of ion-flux limits the etching yield. Etching selectively of 30 and 2.5 are achieved for polysilicon/silicon dioxide and polysilicon/photoresist with Ar and Cl bombardments. The ion energy dependence in etching polysilicon and silicon dioxide was found to scale linerly with (Eion 1/2 – Eth ), where the threshold energies, Eth , are 16 and 40 eV for etching polysilicon and silicon dioxide, respectively. With Ar ion bombardment, Cl was found to dominate the etching reaction over Cl2, as the dissociative adsorption rate of Cl2 was limited on chlorinated polysilicon and silicon dioxide surfaces. Etching of photoresist was strongly dependent upon the heat treatment to the photoresist prior to etching. The angular dependence in etching polysilicon and silicon dioxide was also examined. Maximum etching yield of polysilicon was observed at normal ion incident angle, indicating that the etching reaction is chemically driven and ion-induced surface chlorination compensated for the less efficient momentum transfer to the surface atoms at near normal incident angles. Maximum etching yield of silicon dioxide was observed at 60 ̊ off normal incident angle where the momentum transfer to the silicon dioxide surface was most sufficient; suggesting that the etching of silicon dioxide was physically driven. Possible reaction pathways were proposed and reduced into kinetic models that are usuable in a profile simulator. Cl was utilized to assess the chemical effect of a reactive ion, and simulate accurately a high-density chlorine plasma. Cl further enhanced etching of polysilicon, comparing to Ar, especially at low ion energies or low flux ratios where Cl sufficiently chlorinates the silicon surface and produces volatile etching products. The threshold energy was reduced to 10 eV with Cl bombardment. The effect of deposition of etching products was determined utilizing a SiCl2 beam. The presence of SiCl2 significantly suppressed the etching of the polysilicon by both Cl and Cl/Cl , by almost an order of magnitude. Notching formation at the polysilicon/silicon dioxide interface during overetching is demonstrated to be caused in part by stress enhanced spontaneous etching and is not solely a result of feature charging. Mechanically bending of a patterned polysilicon sample was employed to change the stress of the polysilicon filme without any other modification of the film. The degree of notching was enhanced by the imposed mechanical stress, indicating that the notching formation was greatly affected by the stress, as charging and ion trajectory distortion were not significant in the beam system. A Monte Carlo based profile simulator was constructed to simulate feature profile evolution during plasma etching processes. The etching yield models implemented were formulated from out beam scattering studies in which the functional role of Cl, Cl, Cl2 and SiCl2 were characterized, and dominant reaction mechanisms such as surface chlorination under ion bombardment, surface re-emission, and ion reflection were accounted. The profile evolution of patterned samples etched with the combined Cl and Cl beams were simulated to validate the simulation results. Quantitatively good agreement was found between the simulated profiles and the measured profiles with one adjustable parameter, the chlorine recombination probability on photoresist surfaces. Thesis Advisor: Herbert H. Sawin Title: Professor of Chemical Engineering and Electrical Engineering and Computer Science.