Trenching

Profile trenching is caused by ion bombardment (slide 1). Ions arriving at grazing angles on the feature sidewalls get reflected and accumulate leading to a localized higher etch rate. This effect is well known in sputter etching and in plasma processing. At least two possible explanations for trenching can be found in the literature:
- Ion scattering from sloped sidewall surfaces (S. Van Nguyen, D. Dobuzinski, S.R. Stiffler, G. Chrisman; J. Electrochem. Soc. 138 (1991) 1112 / T.J. Dalton, J.C. Arnold, H.H. Sawin, D. Corliss; J. Electrochem. Soc. 140 (1993) 2395 / J.C. Arnold, H.H. Sawin; J. Appl. Phys. 70 (1991) 5314 and others)
- Ion deflection due to differential charging of microstructures (G.S. Hwang, K.P. Giapis; Appl. Phys. Lett. 71 (1997) 458 / M. Schaepkens, G.S. Oehrlein; Appl. Phys. Lett. 72 (1998) 1294 and others).

Slide 2 illustrates research results on the role of the plasma chemistry on trenching. Severe trenching is observed with pure chlorine chemistry in a bias power range of less than 100 W, while HBr and HCl show little or no trenching. This could be explained by the angular distribution of the impacting ions or the presence of fast protons neutralises the negative charging of the mask in the case of HBr and HCl.

Bogart et al. showed that the microtrench shape and depth seems to be in a first approximation independent on the nature of the mask (oxide vs. resist). Hence charging does not seem to be the primary cause of microtrench formation. They concluded that the angular distribution of ions impacting and subsequently scattering from the etching feature are likely to be the primary cause of non-vertical sidewall and microtrench formation (slide 3).

Schaepens and Oehrlein found microtrenching to be influenced by the direction of a weak magnetic field. This field changes only the angular distribution of the electrons. The ions are too immobile. This result strongly supports the differential charging mechanism (slide 4).

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