Paper PS-TuM11
Effect of Inductively and Capacitively Coupled Plasma Pulsing on Charging of Features in Plasma Etching

Tuesday, October 21, 2008, 11:20 am, Room 304

Plasma charging damage presents challenges to maintaining critical dimensions during plasma etching of high aspect ratio (HAR) features (aspect ratio > 50). In one form of process induced charging damage, charge retention at the bottom of trenches can lead to breakdown as the accumulated charge stresses the material and creates a weak path for the injected current.¹ Charging damage occasionally manifests itself as tapering and twisting of HAR features, where the via or trench turns from the vertical to oblique direction.² This behavior is erratic in nature due to the randomness of the ion and radical flux composition as the feature dimensions approach only a few tens of nm. Polymer deposition on the sidewalls during dielectric etching can trap charge, which leads to less than ideal profiles. Neutral beam etching³ (decreased interaction of charged particles with feature) and UV photon bombardment⁴ (which increases surface current and conductivity allowing charge to drain through) have been suggested as useful techniques to mitigate charging damage. Pulsed plasma operation of an inductively or capacitively coupled plasma reactor may also allow for control of charging damage if negatively charged species can be extracted from the plasma. In this paper, pulsed and continuous plasma operations will be compared for etching in electronegative plasmas using results from a computational investigation. A 2-dimensional plasma equipment model (HPEM)^2,4 has been linked to a Monte Carlo feature profile model⁵ to assess the consequences of pulsed plasma operation on charging of features. Results will be discussed for source and bias pulsing in an ICP reactor for poly-silicon etching and for pulsing of dual frequency CCP reactor for dielectric etching. Pulsed plasma operation allows for reduced interaction of charged species and depending on the mode of operation may allow for electrons to overcome the sheath potential thus neutralizing the accumulated charge. Sustaining a steady pulsed plasma can however be complicated in strongly electronegative gas mixtures as the plasma may not reignite after power is turned off.

¹ T. Ohmori and T. Makabe, Appl. Surf. Sci. 254, 3696 (2008).
² A. Agarwal, M.M. Wang, and M.J. Kushner, 54th AVS Symposium 2007.
³ D.J. Economou, J. Phys. D 41, 024001 (2008).
⁴ K. Rajaraman, Ph.D. Thesis, Univ. of Illinois (2005).
⁵ P. Stout, 60th Gaseous Electronics Conference 2007.