Mechanism of Sidewall Necking and Bowing in the Plasma Etching of High Aspect-Ratio Contact Holes

The mechanism of sidewall necking and bowing during the etching of high aspect-ratio SiO(2) contact holes in a C(4)F(6)/CH(2)F(2)/O(2)/Ar plasma was investigated by monitoring the etch profiles over etching time. As the etching proceeded, the sidewall necking and bowing became severe, and the positions of the necking and bowing moved toward the bottom of the hole. Although the mask slope, which was responsible for the deviation of the ion direction, was already formed after etching for 4 min, the sidewall bowing appeared after 9 min when the necking moved to positions below the mask-SiO(2) boundary. This suggested that the sidewall bowing resulted from the combination of two phenomena: (i) the protection of the top region of the sidewall by a thick CF(x) film deposited at the necking position and (ii) the lateral etching of the sidewall below the necking position. The effect of the mask slope on the formation of the sidewall necking and bowing was examined using a Faraday cage system. The redeposition of particles sputtered from the mask slope on the contact-hole sidewall resulted in sidewall necking, and the secondary etching of the sidewall by ions reflected from the mask slope contributed to the formation of sidewall bowing.